It would appear that gamesas web team took a stab at cutting into the nicknaming conflicts with GameSpy. Some users will still have issues if they own a half-created account which fails to log into gamesas entirely, however if you are able to log into the forums at least, give the following a try:

1) Log into your gamesas account here on the forums.

2) Go to edit profile.

3) Attempt to nominate a new nickname/password (you can also attempt to re-nominate your current nickname and a password however, if you're having problems, I'd assume it's due to nickname conflicts of some sort).

I've managed to fix a couple of accounts this way so good luck. I've posted this in the PC section because I'm uncertain if the consoles are plagued with the same problem (presumably not).

Now hopefully we can start looking at fixing the game itself to revive multiplayer a bit...

Cry-Tom has provided some details in a post here that just went up. If it works for you, please post on that thread to say so!