Golly, whilst away at CTIA, we received a package we weren't expecting. Inside, a brand new battery manufactured by “Celxpert” to replace our stock “Dynapack” battery. Also inside, an explanation:

T-Mobile and HTC are providing a replacement for the T-Mobile Wing battery made by Dynapack at not charge to you due to rare occurrences of the battery overheating.

It's supposedly very rare and all that, but likely enough that T-Mobile and HTC figured that replacing every single Wing battery out there (at least every single Dynapack Wing battery) would be cheaper than dealing with the inevitable burning-pants lawsuits.