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!

Features:

• 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.

Resources:

• Getting Started Tutorial

• [Documentation]: Arduino Pro Mini 328 - 3.3V/8MHz Eagle Files - 2012
• [Schematic]: Arduino Pro Mini 328 - 3.3V/8MHz Schematic - 2012
• [Schematic]: Arduino Pro Mini Schematic - 26 May 2009
• [Schematic]: Arduino Pro Mini EAGLE Design Files