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.

[1]:http://www.raspberrypi.org/wp-content/uploads/2012/04/Raspberry-Pi-Schematics-R1.0.pdf