I was getting this too, just started today. I think it was a conflict between 2 browsers. I was logged in in Firefox, but in IE8 I was not logged in (wasn't even at CBT.com at all). It was getting pretty bad, I'd log in, go to a thread, hit reply and it'd complain that my session had expired or that I wasn't authorized to view the sub-forum I was in.
So far, shutting down both browsers has eliminated the log offs. As an experiment, I've made sure to log into the CBT forums in IE8 too.
strange doings are afoot.