Hacker-Friendly Wireless Phones w/ GPS?

Milo_Mindbender asks: "I'm looking to buy a cell phone that is reasonably accessible to hacking. Not illegal stuff, I'd just want to be able to write and load programs on the phone myself. I particularly want access to the GPS in the phone. I'm not picky about what language or environment I have to use as long no hardware mods to the phone are required and I don't have to pay someone an arm and a leg for a developers kit. Can anyone recommend something (just normal sized phones please, not the big PDA hybrids) they've had good luck with?"