Yes, it’s possible! The Halo DevTeam and Back-of-the-Bus Productions recently launched a mod for the Quake4iPhone game that allows you to play Halo on your iPhone. Currently you can only play a few levels including Sandstorm, Zanzibar, Last Resort, and King.
The Halo DevTeam and Back-of-the-Bus Productions are working on a multiplayer feature, many more levels, and maybe even a campaign mode.
Installation instructions:
1. Install Quake4iPhone from default Cydia source.
2. SSH into your iPhone and replace the Quake4iPhone folder with the one included in this mod, and re-spring
If the installation method above doesn’t work for you, try this other method.