This dude totally just flipped over the envelope, so to speak, of what is geekily possible with the hobbyist-class Raspberry Pi computer. His name is David Hunt and he has built a mobile phone that can actually make calls.
If you own a Raspberry Pi unit (or planning on acquiring one) and are feeling really geeky, he has written a very detailed guide so anyone can build a PiPhone too.
Photo credit: David Hunt’s Blog