As OMG! Ubuntu! reported, Unity for default desktop instead of GNOME Shell is gonna be up for discussions in the next UDS. What do you think? Personally, I'd like to try it on the regular desktop version.
EDIT Unity will be used as the default DE, Mark Shuttleworth has announced at the UDS. More info at Jono Bacon's blog post.
EDIT With the prevailing skeptic reaction about Unity being the default DE, Jono Bacon has posted in his blog to further clarify the issue of Unity replacing GNOME Shell.