One of the first TwentyTens to ship was promptly modified by Phillip Stevens to increase the MCU frequency from the factory 16MHz to 22.1184Mhz! Phillip removed the original crystal and replaced it with a higher-frequency part, and modified the MCU bootloader to operate correctly at that frequency.
See all the details (and a brief review of the TwentyTen) on his blog:
http://feilipu.posterous.com/freetronics-2010-arduino-overclocking-and-rev