I had the same issue for a while, on my windows 7, it seems there are multiple things that may be using the same resources you need for this process to work. Try ctr + alt + delete to view running processes, and shut down anything that may be using adb. It is crucial to shut down Asus sync, asus webstorage, etc, as they tie up your adb line when trying these methods.
I had bricked my transformer as well, and all the processes to fix it weren't working because of "waiting for device" messages.
They finally started working once I closed all other programs using adb.
Plus, with nvflash, make sure to unpack the Rom into the folder, rather than just having the rom's zip in the folder.
After fixing the "waiting for device" error, I still couldn't get mine to work because the rom wouldn't flash over as is, I had to take the "Prime" rom's tar located with in nvflash, and extract the files into the same folder.
Once I did that, I ran nvflash, and the rom transfered over fine and I was able to unbrick my device.
Nexus S 4G - Rooted