The native Maya importer doesn’t work with Maya 8 yet. We are working on it.
If you want to accelerate the process:
Hit reimport on the .mb file, when the error shows up run Report Bug.app.
(This gives us all the log files showing what went wrong in the import process)
also, you don’t have to downgrade, you can run maya 7 and may 8 on the same license, maya licenses are backwards compatible. So just have both installed at once and only work in 7 until unity supports 8.
The folks at OTEE are, as always, right on the ball. They just sent me a tweaked version of the maya importer scripts in Unity, and now everything works fine.