I have the same problem with 2.2 on my Incredible. All of my music are AAC from iTunes, synced with DoubleTwist. The pattern I've identified is that _all_ of the AAC files I ripped from CD (with iTunes) is no longer playable ("unable to play this type of audio file") and marked as "Unknown" artist and "Unknown" album (even though the tags are correct). All of the files I purchased from iTunes online have the correct information and can be played. Again, all of the files are AAC, and the issue isn't DRM protection.
FWIW, it looks like the DoubleTwist app recognizes the tags (has correct artist and album info), but doesn't play those files either.
This is a serious problem for me as the majority of my music collection is ripped from CD in this manner, and is no longer playable.