I'm not sure if you're experiencing the same problem I was, but maybe.
I had updates available I click on Update manager, it opened but never initialized, but said 68 updates.
My problem was that I wrote backports entry wrong.
Backports for lenny and squeeze are listed different, so , if you just edited your sources.list changing lenny to squeeze it's not right.
lenny-backports main contrib non-free
Hopefully this helps.