A USB module may not be so simple to handle. You may need one PCB that feeds power to the module and converts the raw signals (possibly at some evil voltage levels) to a standard USB connector for interfacing with a PC. Then another PCB that instead interfaces a configured module to the microcontroller.

A 8051 is not likely to manage a man-in-the-middle to transparently connect a 3G module on one USB interface (making the 8051 a USB host) and on another USB interface (this time device) let a PC take charge of the module.

And using a serial-to-USB interface for communicating with the USB module will probably only work after the module is already configured to behave as a virtual serial port. The serial-to-USB adapter may not be able to send the magic USB commands required to talk with a USB modem. If the PC only sees a serial port, how will you then get the PC to tunnel unknown USB commands to configure the module? I haven't seen any 100% transparent "USB over serial" solutions where the PC can see an "any type" USB device, but have the commands tunneled over RS232 and then finally end up as USB commands again at the modem side.