I have a similar problem, or at least the same symptoms.
The board is an Arduino Mega 2560. I do not know how to tell a revision number on it, but was sold as Rev 3. So far as I am able to determine it is a genuine product, not a knock-off.
I have tried different PCs, Win 8 (64 bit) Win 7 (64 bit) and Windows 2008 server (32 bit) with the same results.
I have tried both version 1.0.6 and 1.5.8 of the IDE, with the same results.
I have tried the loopback test, with RESET jumpered to GND and TX0 (pin 0) jumpered to RX0 (Pin 1) which appears to fail. I am unsure if I have this done correctly. I do see the RX light flash while trying to upload a program, but not the TX light, even during the loopback.
This board worked for a while, I had programs running on it, and had successfully uploaded several programs on it previously, while working with it, it suddenly stopped communicating with the PC, and has not worked since. I did have a shield board attached, which I have separated, and am therefore only running the arduino on a USB cable.
Given that it worked initially, and that the loopback test appears to be failing, does this mean that my arduino is dead? If so, how can I have any assurance that a replacement is going to last more than the hour or so that this one did?