Actually, I've found out a bit more on what's happening.
I'm kind of lazy when it comes to writing urls, so I write "http://jboss.org", without the "www". What happens on all the links on the top menu besides Forums, is that the url adresses are relative to what I wrote. That is, they all start with "http://jboss.org". But the Forums link is an absolute path with the "www" in front.
So when I write "http://www.jboss.org" I get logged in immediately. Why this is so, I don't know. Maybe I registered myself without the "www" in front.