During and interview with mocoNews, T-Mobile's CTO Cole Brodman confirmed that a second Android phone would be coming sometime in "the next few weeks."
Brodman went on to say that "We'll have multiple new Android devices-from multiple partners-this year." So it is still up in the air whether or not the second phone will be manufactured by HTC, or a new partner like Motorola or Samsung. Still, we are putting our bets on the G2. [mocoNews]
UPDATE: T-Mobile has doubly-confirmed the news saying: "T-Mobile is planning to offer the follow-on device to the T-Mobile G1 early this summer."