N3TKaT has released a great Sega Master System & Game Gear emulator for the DS, heres some info:

0.2: NeoFlash compo version (public)
- Changed to mode 0
- Sprites and background using DS hardware sprites/background
- Added PSG sound

0.1: First version (private)
- All graphics using bitmap mode 5
- No sound



Download Here --> http://nintendo-ds.dcemu.co.uk/dsms.shtml