I'm aware that FluBBa and pepsiman have already written BIOS dumpers, but neither of these work well with modern carts (i.e. Slot-1 carts), and no source was available. So I whipped up my own ARM7/ARM9 BIOS dumper that should work on any cart supported by DLDI:
http://www.cryptosystem.org/projects/nds/biosdumper/
Thanks to Martin Korth for the info on GBATEK, and cearn for helping me with my assembly.
Edit: And right after I posted this I found out about DSBF_dump. Oh well.