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!
The latest and greatest version of this board breaks out the ADC6 and ADC7 pins as well as adds footprints for optional I2C pull-up resistors!
Try this - the super-flat SparkFun Arduino Pro Mini, featuring a full 5V design running a ATMega328 at 16MHz. 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!
Note: Unlike most other Arduino development boards, this one has the USB connection off board.
Getting Started Tutorial