Hmm. I've noticed a lag sometimes. I'll have read a new message, and the main page (I think) will show that I still have a new message, but the forums page shows it as read, ie: 0 new messages.
I belive the lag is due to the fact that the forums main page is part of the message board software, which includes the private message mailbox and other features. So when you read a new PM, it's updated on that page immediately, while the main page is pulling that information from the message board software, which creates a lag.
Regardless, within 5 minutes, the disparity seems to vanish.
André