A couple years ago when the Apple Intel transition was taking place, I sold my kids' two G4 Mac mini's and replaced it with a Intel Core Solo-based mini. The performance has been disappointing from the get go...especially with video (ala PhotoBooth, iChat, etc). I promised the kids I'd upgrade it once CPU prices went down.
For a couple of months, I've been trying to score a T7200 (2.0Ghz Core2Duo, 4MB) for under $250 on Ebay. Nothing doing. So, I just pulled the trigger on a new one from an Amazon partner. So, it will be at home waiting for me when I'm done here in Iraq...and I can finally upgrade the mini for them.
It is amazing how timeless that mini has been. If I were to upgrade the CPU today with the T7200, it would be NO DIFFERENT than the currently-shipping mini's that were refreshed this fall. With the core2duo's being a drop-in replacement for the core solo and core duo's, that has really extended the life of, not only the mini's, but any socketed core duo-based laptop.