Interesting that nobody has actually talked about the claims of the Creative patent. Since the claims are what define the scope of the patent, that's really the only thing Apple is concerned about. These claims are pretty broad--just about any at least three-level hierarchical interface for selecting tracks in a portable media player will be covered. So if the risk to Apple is real (i.e., either having to a) pay up or x-license; b) stop selling iPods; or c) rework the interface so that it does not infringe), then they'll need to take this seriously. By asserting their own patents, it puts them in better position on the cross-license front. (One lesson to be learned here: patents can be used for defense, too)
Also interesting is that the claims are all methods of selecting a track. That means that technically, it isn't Apple who is infringing--it's the end user. So Creative must prove "contributory infringement" or "inducement" of infringement. Each of those have their own legal requirements, which are probably not so tough to meet here, but are hurdles nevertheless.