If you want something hackable go for the Motorola ROKR E6 (or another EZX phone). It runs a linux OS making it easy to modify. It works well as a music player. It has an okay camera. And it has a SD card slot.
I'm not exactly sure what homebrew is but I think it has something to do with emulators. There is a decent GBA emulator for the E6. And hopefully there will be a SNES emulator soon.
Anyways, check out motorolafans.com if you're interested (look in the forums).
See http://www.motorolafans.com/forums/viewtopic.php?t=11859 for the guide for modding the E6.
