I've heard different things about UI consistency on the iPhone (<spit!>). Things pertaining to screen rotation, keyboard usage, even basic "gestures" that vary or are simply inaccessible, depending on the applet you "run".
If slight delays in starting applications and a non-unified user interface are the only price to pay for an open platform, I'll gladly pay through the nose.
Besides, I'm of the opinion that the only thing a unified user interface achieves, is to restrict both what a user can do and what a developer wants to be done.