Update: We've received an official statement from T-Mobile on the matter, and it sounds like they're "giving priority to existing customers" with new inventory coming in. Sweet! Here's the full verbiage:
"T-Mobile understands the inventory challenges for the HTC HD2 and continues to work diligently in order to supply our customers with this highly popular product. As we receive additional inventory of the HTC HD2, we are giving priority to our existing customers. However, the demand for the device is still exceeding our supply.
Depending on the location, some T-Mobile retail stores may still have some HTC HD2 inventory available. We suggest that interested customers contact their local T-Mobile store via phone before visiting to see if they have the HTC HD2 available. If they do not have the HTC HD2 available, some stores are taking names and contact information, and will be able to notify customers once that location has product available."