Just wanted to let folks know that Robert Broglia's new Sega Genesis emulator MD.EMU has just been released on the Android Market. It plays the games that the others never could (Herzog Zwei, Star Control, etc.) and the emulation is spot-on. He plans on integrating Master System compatibility and is hinting that he might have Sega CD games work at a future date. It works great with the Play's controls.
https://market.android.com/details?id=com.explusalpha.MdEmu
https://market.android.com/details?id=com.explusalpha.MdEmu