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 ^[Hide]

[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

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Arduino Pro Mini 328 - 3.3V/8MHz

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Description ^[Hide]

Documentation ^[Hide]

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