Haykuro’s Blog has recently announced that they have successfully ported software from the HTC Magic/G2 to be run on the T-Mobile G1. While the build is being reported as completely stable, you may still experience some issues (as listed on the Google Project site). The installation requires root access and a memory card, and requires knowledge in flashing the ROM on the device. Haykuro has, however, pre-included a variety of worldwide APNs on the software, making it easier for those on AT&T and T-Mobile in the states.
You can find the instructions here.
You can download the builds here.
[via Boy Genius Report]