Buy single core Mac mini, upgrade later.
Yes, if you buy a core solo intel Mac mini at 1.5 GHz now, you can already pop in a 2.16 GHz Core Duo if you want. At 700 USD, that processor isn't exactly cheap, but one day, it will be. Or you'll buy an 1.83 GHz Core Duo then. Or the owner of an 1.67 GHz Core Duo mini will upgrade to a 2.16 and sell you his old part for cheap. Either way: Here's the xtremesystems.org
forum thread about it with pictures.
It seems like this makes the mini an even better upgrade-path machine than before. Want a fast intel Mac for cheap? Get a mini, upgrade it later.