I imagine that iPhone OS v3.0 has been in the works for awhile, far before the WebOS was unveiled. I bet there are similarities (especially if they revamped the Springboard app) but other things were just being developed while Palm was doing their thing.
I can't see folks at Apple seeing the Pre demo and scrambling to rip-off a bunch of stuff in the remaining months of development.
The big thing with the Pre is the card-like metaphor for navigation. I doubt Apple will steal that for the iPhone, though it might be possible that we see a command-line launch system similar to the Pre's.