As others have implied, I have a sneaky suspicion that on an Intel Mini or even iMac (both of which have socketed processors), this might not be as straightforward as it is in the Windows world. Aside from that upgrading the processor in a Mini would be similar to adding a jet engine to a VW bug (not saying it wouldn't be neat...), there's a few things that might need to be solved first: firmware compatibility, memory controller compatibility for exceeding the 2GB of currently allowable RAM in these machines, OS recognition (would popping in a new processor require a complete re-install to prevent KPs?), and possibly cooling (though I don't know much about this in the Intel world). Heck, Apple might even install a firmware block like it did in the old G3 B/W towers...
There could very well be a new market for the same old upgrade players in the Intel Mac world (Sonnet et al.) to supply these chips and the necessary system modifications for folks to install them. Heck, these folks could even get in to the G5 upgrade market as well if they wanted - it's a real long shot, but maybe in a few years they'll offer replacement dual-core G5 midplanes for PPC iMacs...
Now I'm really hoping against hope that future Apple Intel laptops have socketed processors as well.