Just for fun I programmed via the ICSP interface with the ASCII table sketch, set to output at 9600 baud.
At 20 MHz it output (seemingly correctly) at 12000 baud which sounds right (9600 * 20 / 16 = 1200).
Then I put in one of the 24 MHz crystals that I also ordered (because dhenry claimed you could clock it at 24 MHz) and it output at 14400 baud (9600 * 24 / 16 = 14400). So on the face of it, you can run at higher speeds. Now I can't say for sure that it is working perfectly, but after running for 10 minutes or so, the ASCII table is still coming out.
Trap for young players: I initially saw no output, which I spent 15 minutes mucking around trying to fix, until I realized the sketch only outputs the table once. At 24 MHz that doesn't take long!