T-Mobile's new Google Android phone, the G1, is not as open as you'd hope -- all the good hardware is sandboxed off from the development environment and requires a signature to run. But hackers are already working to crack open the firmware. From the #android channel on Freenode:

I hacked my camera's firmware manually by using an exploit to cause it to execute arbitrary code - and then blinking out the entire firmware in 0's and 1's on the autofocus LED - read in by a photo transistor attached to a sound cable plugged into my microphone port - and then put back into 0's and 1's...
Then disassembled the ARM9 code in it and worked on porting CHDK to it...
I'm pretty sure having a whole OS at my disposal should make this a lot easier

The T-Mobile G1 — nice phone, but not totally open