I've had a look at the [schematic][1]; the data pins for the micro USB connection are not connected to anything. Therefore, there is no way to mount the RPi as an external device.

It could be possible to mount the RPi's hard drive or login over the network by using SSH.

The GPIO pins include a set of UART data lines, which could be used to form a serial connection to the RPi from your PC.

### Bit-banging USB
Bit-banging USB has been done ([AVR V-USB][2]), however, you would need to design your own expansion board that added another USB port. It is conceivable this port could power the RPi. Having done this, you would need to write some sort of driver. Good Luck.

If you are prepared to go to all this trouble, it might be worth looking at an [FTDI Chip][3] instead. You would connect this to the UART pins of the RPi. From a software perspective, you would communicate with the computer as if it was a serial device.

[1]:http://www.raspberrypi.org/wp-content/uploads/2012/04/Raspberry-Pi-Schematics-R1.0.pdf
[2]: http://www.obdev.at/products/vusb/index.html
[3]: http://www.ftdichip.com/