I have just posted a news item on a free online book I found, and it's all about development for the GameBoy Advance.

With a little work and the latest sources of Free Pascal it shouldn't be too hard to get a few GBA demos piped out. For hardware you can easily use an emulator like VirtualBoy. Some minor changes in the op codes for ARM7 would need to be adheired to, but other than that you're pretty much good to go at some point.