Chalk this up to dubious, but according to Tom's Hardware, sources at an nVidia partner have said that Apple has a Mac Mini refresh sporting nVidia's Ion platform in the works. Apparently Apple had some very early prototype systems, before many other partners, no doubt partly by dint by of its collaboration with nVidia on pushing 9400M out, via the recently updated MacBook range. Assuming such a development is happening at all, of course.
Other sites' sources, though, conflict with this report, suggesting that ION will actually see its way into Apple TV boxes. That makes a lot of sense, as the relatively low processing power but very small form factor is a perfect match for the HD media streaming box. Especially with the 9400M chipset's excellent HD decoding capabilities.
That Ion would appear in refreshed Apple TV boxes, rather than Mac Minis, definitely seems more likely. Yes, nettops have their place, but with rivals such as the Dell Studio Hybrid slowly offering compelling (albeit Windows-based) alternatives, Apple surely needs to compete with systems offering similar performance.
There is a possibility, though, that both Atom and Ion-powered and regular Core 2 Duo (or even quad!) Mac Minis could be launched together, obviously having a reasonable pricing gap between them to differentiate the performance difference. Supposedly the new Mac Mini will be launched in March, so feel free to speculate wildly until then.