I experienced the same 2 days ago. I just logged-in on gmail from my computer (i never use this account...). After some hours on my N900 all Gtalk contacts lost online status. My contacts can see me online but i cannote see them.
I tried this:
-remove GTalk account
-delete all Gtalk username from all my gtalk contacts
-add again Gtalk account
After that i connected the account but the sync didn't happen.
Tried many times but nothing.
I'm thinking that the only way is to flush N900 and try again...
Any ideas or clues???