I agree EowynCarter. Mobi will eventually die out, and Amazon will be the lone island in a sea of Epubs.
There's another reason I think you'll see Epubs win. For one, the more content a user can get, the more likely you are to be able to sell your ereader. No needing four different ereaders for four different services. Plus as technology improves, backwards compatibility will make your older ebooks still work on your newer devices. People love that kind of stuff.