There is an Epic Level Handbook that can be used for adventures over 20th level.
It gives you treasure value and exp and such for encounters above CR 21. It looks like for treasure it sticks to purely value of worth.(GP) Rather than actual items, since Epic magic items are worth alot more than the normal items.
Also, the Dungeon Masters Guide Gives you additional tables and rules for up to 30th level.
I cant copy and paste specifics, but the DMG has info around page 52 and the Epic book had stuff around page 121ish.
EDIT(As suggested by comments below)
The epic level book is a bit more precise, by just using the total treasure value. The DMG simply suggests adding more magic items per CR. The Compendium specifically talks about replacing the random tables in the DMG with these, so I would even go so far as to suggest the expanded CR table in the DMG(on the bottom of page 52) is the "proper" course of action.