I think you might be confusing Aero and Avalon. Aero is the longhorn UI. Avalon is a new presentation stack. MS originally said Aero will be built using Avalon technology, however I find this difficult to believe now.
If it’s “under debate” as to how Avalon will ship, the LH UI (Aero) can’t be built using Avalon because worst case scenario Avalon won’t be there.
That’s not to say that Aero / LH won’t use Direct-X and hardware acceleration as you stated.