So, guess what? I'm in Europe and playing XCOM right now! (Also Dishonored, but that can wait until I save Earth from the alien menace)
Here's how to do it:
Go here: link
Sign up for their free 3-hour trial. Once you've got your login and password, sign in. You'll be sent to the user start page. Download the Multiprotocol VPN tool under the recommended downloads.
Once that's installed, run it. You'll see a window with a server list. Pick a server in the US or Canada (I used USA-NY, but I suppose the others would also work.) Connect to one of these servers.
Change your Steam download region in the Steam Settings under Downloads. I actually changed this after I started running the VPN, but Steam didn't care. Might want to do this first though, just to be sure.
Close Steam and restart. Once Steam restarts, it should start unpacking XCOM. Wait until the game is done unpacking and start it up with the VPN running to make sure it works.
If it works, exit the game, select Go Offline in Steam (It's under Steam in the top left). You can close the VPN now too. You can go online in Steam again once the game officially releases in Europe.
There, now you can play XCOM! (and perhaps Dishonored, you should play that too)
And before you ask:
No, I'm not going to get banned from Steam. Valve doesn't care if you use VPN's to unlock games early, they only start caring if you use VPN's to purchase games cheaply through Steam, by pretending that you're in Russia or somewhere else.