T-Mobile and HTC haven't been very nice to their customers. They decided they should try to lock down their phone much better than before, which pissed off quite a few developers willing to root the device at first.
The G2 would revert back to its original version of Android, as soon as someone tried to root it in the beginning. Fortunately, this was fixed by a developer later, who managed to make a permanent root for it. Later he was also able to unlock the phone for every GSM carrier. The lock that keeps the firmware from being flashed to work on another carrier was also disabled, along with the signature-based security check of the phone's firmware that makes sure it's authentic.
Now that all locks have been broken, you'll be able to keep your G2 up-to-date for as long as the hardware can keep up with new versions of Android.