The Android-based G1 has already started showing up on T-Mobile USA's website, as a promotions page for existing customers. While the pre-registration link does not yet work, it's apparent confirmation that the device will be offered to existing T-Mobile users prior to general availability.
It also confirms the handset's name - the T-Mobile G1. Up until now the device had been referred to as "T-Mobile's first Android-based device".
There is still no word on pricing; the current rumors suggest $199, but that may not necessarily refer to the pre-registration price for existing subscribers. T-Mobile have scheduled a launch event for the G1 on September 23rd, which Android Community will be Live Blogging at http://Live.AndroidCommunity.com/