Adafruit has announced "Gemma," a bite-sized, Arduino compatible board intended for use in wearable electronics projects. It measures 1" in diameter, and while it's not shipping yet, they're taking names for people who want to get 'em when they ship:

* Powered by the ATtiny85 with 3 available I/O pins, one of which is also an analog input and two which can do PWM output*
Progammable over the micro USB connection*
Onboard 3.3v Regulator and power LED*
Reset button*
Works with our Flora NeoPixels (can drive about a dozen - not much RAM!)*
Super tiny design, only 1" (25mm) diameter & 4mm thick

Adafruit Gemma - Miniature wearable electronic platform (Thanks, Matthew!)