I've finally gotten round to buying this today (retail DVD of the original release) but the CD key that comes with it is already being used by someone else!

I'll try to get another copy from Game tomorrow, but if that doesn't work out what other options are there? Is there a way of bypassing the need to register the key, or maybe a way to avoid Steam altogether?

nope and you shouldn't have to, if you have a legit copy, don't stand for having to hack your way round a system. You can but in lots of continued effort for a product you legitimately own, but for a retail copy you need to sign in to steam and get the content valididated on steams servers.

I suspect, someone has stolen the cd key when they went in the shop? If it was on display, take it back and ask them to get one from in storage.

You could also try contacting valve but i suspect they will be friggin useless. I would send off to valve and tell them someone is illegitimately using that key regardless.

Valve will help actually!
Go to these forums: http://www.chatbear.com/board.plm?b=4989&v=flatold

Or try this too: http://www.steampowered.com

It has happened alot, with people illegally hacking the game taking valid keys. Oh and if you get that figured out, when you are installing the first time, remember to install CSS too, or it will freeze. Why?... dunno, but it happened to me and a buddy of mine.

CSS is better anyway...

Game gave me a new copy, and thankfully this key is valid.

I bet they put the one I originally bought back onto the shelves again, I pity the poor bugger who buys it!