Macenstein is reporting that a hacker with access to a Core 2 Duo "Merom" CPU clocked at 2.16 GHz has successfully upgraded his Mac Mini. According to the hacker's benchmarks, the upgraded Mini beat out a Dual 2.5 GHz G5 in an iTunes encoding test by 15%. Note that these benchmarks could not be confirmed by MacRumors, but are placed on Page 1 for interest's sake.
Both the Mac Mini and the iMac have socketed CPU's, which make them suitable for swapping with Intel's imminent Core 2 Duo chips, which are conveniently socket-compatible with Intel's current Core Duo chips. The MacBook and MacBook Pro however have their CPUs soldered to the motherboard, making upgradeability impractical.