The G1 has already received Android 2.2 thanks to some hard working hackers, but there is still an outside chance that T-Mobile could upgrade the device over the air. Before I get your hopes up, just remember T-Mobile has the final say on firmware upgrades and their official response for the last couple of months has been, "We will let you know when we have more details to share".
Buried inside the Froyo code drop post on the Android Developers Blog was the news that Google included all the configuration files necessary to build the source code of Android Open-Source Project on its own. Android engineer Jean-Baptiste Queru noted that, "You can now build and boot a fully open-source system image out of the box, for the emulator, as well as for Dream (ADP1), Sapphire (ADP2), and Passion (Nexus One)."
As many of your know, the G1 is the HTC Dream and the exact same hardware as the Android Developer Phone 1. Some developers have already reported success in building Android 2.2 for the Dream and we know the team behind CyanogenMod is also working on a port.
What I find most interesting is that those who built Android 2.2 from source were able to flash it on a Dream using any version of the secondary program loader (SPL). The most recent crop of custom Android 2.1 ROMs for Dream have required the DangerSPL, which rewrites the system partitions to address the device's limited internal storage. Carrier builds are always larger than the builds from source code (because of all the extra apps), but it sounds possible that the G1 could fit Android 2.2 without modifying the SPL.
At the end of the day it's T-Mobile's call, but I don't see them leaving behind the G1 owners just yet. Even though it is the oldest Android phone, the G1 appears technically able to run Android 2.2 at an acceptable performance level. In the mean time, keep an eye on HTC's Developer Center in case they release the Android 2.2 system images for the ADP1.
Surely T-Mobile would have broken the bad news by now if they did not intend to upgrade the G1, right? Do you really think they kept you waiting all this time just to disappoint you?