Until T-Mobile, HTC and/or Google get their act together and release a 2.x upgrade for older phones, your only option is rooting your phone and installing a custom ROM.

Here are some guides for rooting a G1 or MT3G. This is a straight-forward process if you are very careful and follow directions to the letter. Once your device is rooted, you can proceed to install an advanced Recovery image, and then have your pick at various 2.x ROMs available here (do a search for "eclair").