Update 5/31/12:  It’s over a year since I posted this originally, and I’m still seeing comments coming in that this is still a problem.  Not very encouraging…

Because people are still having problems with this, below I’m going to copy the comment from the Steam forums talking about how to deal with this, so you don’t have to hunt around for it.  The following procedure did work for me to get Steam to update eventually, and once you get it up-to-date now it seems to use a different update mechanism that has, at least so far, been more reliable.  The following instructions are from Steam forum user RoyalSFlush, and I take no credit for them whatsoever (there’s also a link to the whole forum thread further down in this article).

I had a similar problem, fixed it thanks to Lavec’s post, here’s what I did:

Open up your Finder, Applications, Utilities, Terminal.
Terminal should launch – if you’re not familiar with it, don’t worry, do exactly what I say and you should be fine.
Type:
cd /
cd Applications
cd Steam.app
cd Contents
cd MacOS
./steam.sh

That should run Steam and it’ll begin updating, exactly as you’d tried before, except this time, you have the log of what’s happening on your terminal. Let it run until you get the error. Close the dialog box and look at the terminal log, you should see this message:
“SteamUpdater: Error: Steam needs to be online to update. Please confirm your network connection and try again.”
Somewhere above it there’s “Failed! (some link that starts with http://cdn.store.steampowered.com/)”, copy the link and paste it into your browser – that should download the file. As soon as the download finish, move the file to Steam.app, that is: open Finder, Applications, control-click Steam icon and select “Show Package Contents”. It will open a new window with a single folder named Contents. Open it, the MacOS, then package. That’s where the download file should go. After putting it there, you can restart the installation double-clicking the Steam icon or using Terminal (if you still have it open, just type ./steam.sh again).

Thanks for the excellent idea, Lavec, I wouldn’t figure it out if it weren’t for you =) Hope this helps, guys!

——

At some point, I should probably write a review of the various digital download services – after all, the frustration with a game can begin even before it arrives, due to issues with downloading it in the first place (and buying it on a disc these days is no guarantee of avoiding this, either – most games require online authentication and/or downloads of various “bonus” content).  [Here’s a quick summation, though: Steam is good if you can actually load it up (more on that in a bit), gog.com is almost flawless with a nice, simple downloader, Direct2Drive has awful download clients that have sometimes eaten/restarted downloads midway through, GamersGate is reliable but I’ve had issues with slow downloads from them, Impulse previously had a nasty habit of vanishing downloads in progress when it updated itself, and Games for Windows Live is a near-incomprehensible mess of a system that I can’t stand to use, although when you get to the point of actually downloading a file, it usually goes okay.]

Today, however, I’d like to talk about Steam, specifically Steam for Mac.  For quite some time, there’s been a bug I’ve experienced during downloads that has crippled Steam, and made me dread each notification that a new patch for it has arrived.  Simply put, when Steam starts up and finds a new update, it begins to download it, and won’t launch itself until it has finished downloading and applying the update.  Problem is, the update will often fail to download one of the update files, pop up a message about being “unable to connect to the internet” despite the internet connection being fine, and quit out of Steam.  Subsequent restarts of Steam lead to the exact same issue: load, try to download, fail, quit.  This means that, due to this update, you can’t load Steam at all, or play any of the games installed via that system.

This is, quite literally, a game-breaking bug, and it has been occurring with Mac updates for many months now – out of all the updates this year, only one has updated cleanly the first time without hanging.  Sometimes the issue will resolve itself in a day or two, sometimes it will take weeks without a more proactive intervention.

If you’ve found yourself in a similar situation with Steam, there is a workaround that does seem to work – at least it has resolved the issue for me on this latest update, and can be found on a thread in the Steam forums (http://forums.steampowered.com/forums/showthread.php?t=1566830).  It’s a fairly technical fix (requiring the use of the command line), although a post further down in the thread explains it more clearly.  However, it’s not a perfect fix, and requiring users to go to these lengths just to be able to play their purchased games is ridiculous.

I would strongly encourage Steam to come up with a genuine fix for this issue sooner rather than later (heck, just downloading the update in Steam’s download manager while the application is running would probably help, as even if the update failed you’d still be able to play your games).  That being said, though, it also illustrates the wider problems with having a “gateway” application required to play your games – if the gateway application fails, then every other application that makes use of it becomes unusable.  If you are going to require an application to work like this, for DRM or other reasons, you can’t let it have a routine failure mode that prevents it from launching, thereby disabling tens or hundreds of other applications.  As it is, this state of affairs is shameful, and it is somewhat disturbing that Valve seems uninterested in resolving this issue, despite multiple and continuing reports of problems on its own forums, answered by one post from a Valve staff member, last year, that failed to resolve the problem.  I mean, really, guys, you’d think there would be an issue when you launch a big new game from your own development team, and a number of people can’t even launch your distribution platform to buy it.  It’s time to actually fix this issue.