You mean the sub $400 computer that Apple sells, but can't "really" sell because all sub $400 computers are crap and Apple can't possibly make one? LOL
Yeah Atom and Ion2/Ion might make sense. Intel isn't going to make that ancient Pentium forever (some older chips are going to start to get dumped with the upcoming new branding effort) . Neither is Nvidia going to make that ancient GeForce either. More likely Ion/9400M since it will become "old" and hence cost less. Plus Apple would have tons of experience with it at that point (can reuse all the optimized code already built).
Assuming that the low end Atom's price drops to that of the ancient Pentium M 1.0 GHz they are using. Also assuming can get inexpensive component out with the 9400M graphics abilities. In short, if the component costs are approximately the same, they'll do it.
Also don't think they'd do Ion2 because more folks probably would hack them as "even less expensive" Mac mini's.
However, if Intel adds even basic hardware support for decoding HD class video they also could be a player for this device.