A

There are 3 possible maximum levels:
An imposed maximum by Valve would cut off leveling. Thus far, Valve has not specified any imposed maximum. Based on questions I've asked Steam Support, they are very clear about what has imposed maximums(friends list) and generally don't say anything about what does not (wish list) unless directly asked. It seems likely, based on the evidence available, that there is no imposed maximum on Steam Levels or XP.
A maximum due to technical limitations such as memory capacity allocated to store a value and/or to display a value. Because levels and XP never go negative, it is likely stored in a whole unsigned value such as an integer. An unsigned integer can hold 2^32 − 1 values so 4,294,967,295 would be the maximum amount of XP that is able to be stored in a 32-bit integer. Using some math, we end up with enough XP available to reach level 29306 with the values that can be represented in a 32-bit integer so that is the likely technical maximum unless they used a 64-bit integer.
A practical maximum is a restriction on what is achievable with the resources available. Obviously as Steam gets new badges, cards, and other resources, and as badge values change, these numbers will fluctuate. Referencing various steam accounts and other information to determine Game collector badge values and this handy chart in addition to those accounts for the others. Here are the resources available:
Years of Service Badge XP: Steam was started September 12, 2003 and this badge grants 1 level worth 50 XP per year of service so the maximum at this point in time is level 10 and therefore 500 XP.
Game Collector Badge XP: Steam search data is very unreliable in general and determining what games count is very difficult. Steam search lists a total of 2583 "games" available, but some aren't actually games, some are duplicated and some are missing from the results. It's unclear what counts towards the badge anyways. Without more information, 2410 are confirmed to count towards the badge - close enough to search results until a better determinate is found. All badges above 250 grant 250+#ofGames XP so 2660 XP
Trading Card Badge XP: Each regular trading card set provides for up to 5 levels, each worth 100 XP per regular badge, and 1 level worth 100 XP per foil badge, meaning each set of trading cards is worth 600 XP. Steam search is unreliable to determine which games have trading cards because of the same problems above and the addition of unreleased items for which trading cards cannot be collected. Sorting by date filters out Wake, Lunnye Devitsy, and Dead Island. Even worse, not all games with Steam trading cards appear properly with Steam's search filters as Age of Empires II HD, Age of Empires III Complete Collection, Call of Duty: Black Ops 2 (has 2 badge sets - Multiplayer and Zombies), Call of Duty: Ghosts, Cortex Command, Jagged Alliance: Back in Action, Eversion, Mabinogi, Metal Drift, No More Room in Hell, Offspring Fling!, RIFT, Satazius, Sequence, Stacking, Street Fighter X Tekken, Talisman Prologue and maybe more are not properly pulling up in the trading card category, despite having trading cards. There's also the addition of the software RPG Maker VX Ace which also has trading cards. The formula works out to something like:
(#ofGamesWithTradingCardsOrderedByDate - #ofEarlyAccessGamesWithTradingCardsOrderedByDate + #ofEarlyAccessGamesWithTradingCards + #ofGamesErroneouslyFilteredByDate + #ofSetsMissingFromTradingCardCategory + #ofSofwareWithTradingCards + #ofSaleBadgesFollowingRegularTradingCardLevelRestrictions) x 600.
Filling in the above for the current known values, we get (467 - 26 + 31 + 3 + 19 + 1 + 1(Steam Summer Getaway)) x 600 = 297600 XP
as Black Ops 2 already has 2 sets and there has been mention of more series of cards to come, it seems likely that this calculation will become much more difficult.
Holiday Sale 2013 Trading Card Badge XP: This trading card badge and foil exceeded normal trading card crafting limitations with no maximum level. In theory, it could have been possible to reach levels into the millions were money and time no object, but the practicality of it is that the known maximums reached were 2013 for the regular and 100 for foil to a maximum of 211300 XP
Community Ambassador Badge XP: With a maximum level of 2, with each level worth 100 XP, this offers 200 XP.
Trading Card Beta Badge XP: Is worth 100 XP
Steam Community Translator Badge XP: is worth 100 XP
Steam Hardware Beta Candidate Badge XP: is worth 150 XP
Steamworks Community Developer Badge XP: is worth 500 XP
Steam Community Moderator Badge XP: is worth 500 XP
Valve Employee Badge XP: is worth 1000 XP
Steam Holiday Sale 2012 Badge XP: is worth a maximum of 100 XP. Value per task is used here.
Steam Summer Sale 2012 Badge XP: is worth a maximum of 100 XP. Value per task is used here.
Steam Holiday Sale 2011 Badge XP: is worth a maximum of 200 XP. Value per task is used here.
Steam Summer Camp Badge XP: is worth a maximum of 200 XP. Value per task is used here.
The Potato Sack Badge XP: is worth a maximum of 200 XP. Value per task is used here.
The Great Steam Treasure Hunt Badge XP: is worth a maximum of 100 XP. Value per task is used here.
This gives us a current grand total of 500+2660+297600+211300 +200*4+100*5+150+500*2+1000 =
515510 XP
This is probably the long way to calculate it, but using the arithmetic progression formula for every 10 levels to determine the number of times the experience required for the first 10 levels (1000) needs to be multiplied, taking a look at level 310 (based on where palmdesert's level + some trial and error) gives us (31/2)*(2+(31-1)) = 496 and 496 * 1000 XP = 496000 XP required to get to level 310 and each level after that cost 3200. This leaves us at level 310 with 19510 XP remaining. 19510/3200 = 6 r310, meaning level 316 is the maximum that gives us.
The practical maximum is somewhere around level 316. Valve employees are unlikely to have many of the community participation badges, etc. so not being a Valve employee is likely to net more points in reality. This means 1000 less from what's available and leaves the total available XP just under the threshold for level 316, meaning that the current practical maximum Steam level is probably about 315.
Steam search data inconsistencies make it impossible to be sure how much higher a level can actually be reached because there is no reasonable or consistent way to determine which items are being incorrectly added to or removed from the result sets. Given the unbounded nature of the Holiday 2013 trading card badge, it is entirely possible that an actual maximum level could have been reached, but that is simply impractical.