In the last week of July a number of T-Mobile subscribers began observing that the battery life of their HTC handsets had fallen away dramatically. Both my wife and I have the G2 (aka HTC Desire Z) and have found that typical battery life has dropped from 8-10 hours to less than 4 hours on a charge.
In fact, I noticed that the back cover of my G2 was warm to the touch even as the phone was sitting idle all morning. Even in a completely idle state the phone was drawing enough current to make the battery warm.
There’s a long thread about this issue in the T-Mobile support forums. Over the past few days others have noted the issue in various places, including; Phone Arena, T-MoNews & Phandroid.