you have functions inside your theme that are not compatable with wordpress 2.8 — lots has changed. that OR you have functions that are calling plugins that are not currently loaded.
My guess is that its a little bit of both.
Lots has changed from 2.0.x to 2.8
if you upgraded properly, you have a copy of the defualt theme that came with 2.8 — change to that theme. Does everything display?
If so, youve confirmed 1/2 of what I just said.
start by going back to where you got your theme and see if the author has out out a newer version.