This looks like an interesting uC:http://www.atmel.com/devices/ATxmega384C3.aspx
This thing has a max clock speed of 32 MHz, and also supports A/D, SPI, I2C, UART and so on.
I find this interesting because of the 32 MHz max clock speed. Since it is a power of 2, I am thinking that the Arduino libraries could easily be made to work properly on this uC. Since it has all the features of an Uno or Mega, it seems that a double speed Arduino could be easy to make.
Also, it has 384K flash and 32K SRAM, which is huge.
And a USB transceiver built right on.
Only problem is, I can't see anyone who has any stock on it...