Is there any specific reason the USB pull-up resistor is pulled up through a transistor? I can't see why it could not be pulled up directly by a port pin.
Upon RESET, the port would be high impedance and the resistor thus disconnected until the port pin is driven high and made an output.
The transistor allows to re-enumarate the USB: it offers the possibility to disconnect the product to the USB and to reconnect it with a new USB description.
This schematic have been found on a ST evaluation board.
