You need to wedge an Arduino into a really tiny spot? Simple - just use a SMD '328 IC. Oh... you want a reset, and voltage regulator too? Why didn't you say so!
Try this - the super-flat SparkFun Arduino Pro Mini, featuring a ATMega328 at 8MHz on 3.3V. Mind you, there's no headers and USB interface - you have to wire it up yourself, and have an offboard USB-serial cable to program it with!
- ATmega328 running at 8MHz with external resonator (0.5% tolerance)
- Low-voltage board needs no interfacing circuitry to popular 3.3V devices and modules (GPS, accelerometers, sensors, etc)
- USB connection off board
- Weighs less than 2 grams!
- Supports auto-reset
- 3.3V regulator
- Max 150mA output
- Over current protected
- DC input 3.3V up to 12V
- On board Power and Status LEDs
- Analog Pins: 8
- Digital I/Os: 14
Note: Unlike most other Arduino development boards, this one has the USB connection is off board.