Just a heads-up that Teensy my favorite small form factor Arduino compatible board, just got a solid upgrade with the 32bit ARM Cortex-M4 48mhz based Teensy 3.0. The Teensy 2.0 has pretty much been my bread and butter board for homegrown experiments (and some work related ones also). Looking forward to playing with the capabilities of the new one. The creator also aims to keep Arduino compatibility with the new ARM chip, if you don't want to talk directly to the hardware.
More information about the Teensy 3.0 should become available once the Kickstarter pre-order is completed and the Teensy 3.0 becomes official in the PJRC.com store.
But a quick highlight of the specification are: