In theory, my old G4 (upgraded to 1800Mhz) has more potential, but it's noisy and seems subjectively slower. And it can't run the iPhone SDK!
This little intel 1.6 Ghz Atom is quiet, consumes next to zero power, and will soon be living the iTunes test to see what it can do.
I really wanted Apple to announce a hot new Mac Mini at their show, but since they didn't, this should do for a while. I'm afraid to run the system update to 10.5.6 on it, but if it proves unreliable, I'll just make it into a Fedora or Ubuntu box instead.