I've had quite a few problems with the forums lately (and a few other Live ID sites).

At one point I couldn't open the forums at all, on several of my PCs. IE7 would just show me a white screen whilst continually "redirecting" back and forth between a few URLs.

Another time I could get into the forums, but doing anything that related to me as a user (eg creating a post or viewing my threads) brought me to the "you have successfully signed out" page.

On both occasions, finding the cookie for forums.microsoft.com and deleting it fixed the problem.