Is the Half-Life 2 EULA illegal? on the Inquirer (thanks Ant) describes possible legal issues surrounding the End-User License Agreement for Half-Life 2 and the Steam service. Here are a couple of hefty excerpts:

...The German Consumer Association has recently found that the packaging on Half-Life 2 is misleading. In a report made following complaints from the public, they said that the mere listing of an internet connection under the 'other' category in system requirements did not accurately describe the true extent of the internet tie-in with the game, and ordered Vivendi to amend the packaging and untie Steam from HL2 or face a hefty fine. See this page. How far other consumer associations will agree with the Germans is yet to be seen, but it seems a no-brainer that Steam should be mentioned on the retail pack.

The return of software has traditionally been a bugbear for gamers. Most shops, at least in the UK, have a policy not to allow the return of opened software because of piracy risks. However, most consumers are not aware that, in some cases at least, this is in breach of their statutory legal rights, which cannot be infringed. This page at the UK's Office of Fair Trading conveniently notes an example of when goods can be returned as faulty:...

It is quite conceivable that any gamer not being able to connect to Half-Life 2 is entitled to a legal refund in the UK. The case would hinge around whether or not the inability to play the game without persistent net connection, or the previous hacking of the CD Key rendering it unplayable, makes the game unfit for the purpose described on the box. This could well be a winner.

The last issue is the most interesting and relates to a number of other cases that have come up over the last couple of years. According to US and UK law, under the principle known in the US as 'First Sale', a consumer buying a game takes absolute title to it; that is, they own it. ...

But, arguably, you can't transfer the CD Key without paying the $10 Steam charge, because otherwise it's still registered to you at Valve, meaning you haven't truly transferred ownership to the buyer.

However, here's something else. No mention of the Steam software is made in the EULA for Half-Life 2. This means that the terms of the Steam EULA that you agree to when installing that software are almost certainly not incorporated into the contract you agree to when installing Half-Life 2. Consequently, the $10 transfer fee would not be enforceable because it isn't in the HL2 contract, and Valve would be acting illegally in blocking any sale of the game from one person to another.