If you want a truely fast yet small mobo then go with the Shuttle SB51G barebone setup. The board itself is about 2 inches longer than the mini ITX yet supports the latest Pentium4 3.06 with Hyper-Threading.
Another small board is the FV25 which is less than an inch longer than the mini ITX. It supports the P-III Tualatin CPU up to 1.4Ghz.
Fastest production MiniITX board would brobably the M 9000 Via C3 900Mhz.