According to an AndroidGuys.com reader, who claims to have spoken with a T-Mobile supervisor (presumably in the call center), T-Mobile is planning to send out a normal capacity second battery to its customers, not the larger battery that has been reported quite often recently.
Comments on that post show that users have had mixed results with T-Mobile on the G1 battery issue so far. One person that read the T-Mobile memo says that his interpretation is that batteries can be sent out for any phone that does not perform up to spec, so he sees nothing different here. Others are claiming that they have been sent a second standard capacity battery by T-Mobile, and one says that he was referred to HTC who promised an enhanced battery as a swap for his original.
So far, nothing is clear, and it is looking like we might not be seeing extended batteries showing up in the mail, as we had been hoping for.