Know any electrical engineers who make their own circuit boards?
Then you can make it really small with SMD chips. 
The mini uses the SMD 168 chip.
Atmel chips are cheap and common which is exactly what your looking for.
All the chips in the line are fairly similar so with a little work you can program basically any of them.
The 168 should be able to handle nearly anything you want to do though.
I dont think anyone has posted their code for other processors yet.
Look at the schematic for the Arduino to see how to implant the chips elsewhere.
As MikMo said its pretty simple.