After receiving an invitation to SO Careers I have tried to create an account and login afterwards. Whenever I try to login, however, I get an error message "Oops! Something Bad Happened!" that is not really useful in identifying the source of the problem. It seems to be a different issue than other members report.
Update: It seems other members experience similar problems, see e.g. this post.
I have experienced the same problem on Firefox 4.0.1 (MacOS X 10.6.7, all latest patches installed) and Safari 5.0.5. The problem persists for more than a week now.
Final update: After a couple of iterations, the problem has been solved (although the source has not been fully determined).