You DO NOT need to be on a stock rom to run this and it will NOT erase your data. But it is recommended when you get into CWM recovery for obvious reasons.
If anyone is having any issues and you get caught at the Google splash screen. You can use Power, Vol - to hard shut down the tablet, then Power, Vol +, Vol - to get into the bootloader. From here you can install the factory images from google. Once you're back up and running, you can run this tool to gain root and CWM back.
Updated 11/14/2012: Replaced SuperSu.apk with the latest version designed for 4.2
Updated 11/14/2012 14:41EST: This package will now contain 3 different runme files. One of them will not erase your data, one will erase your cache only and the last one will erase your data and cache. It seems people coming from some roms are having bootloops or hangs because of a lack of data wipe. You can attempt to run the one with no data wipe, but if you run into trouble, try one of the others. Some have made it through with only a cache wipe. See what works for you.
Updated 11/14/2012 16:53EST: Removed full data erase option as I forgot it will erase your SD card and therefor erase the root zip before it gets a chance to flash. There is only 1 runme file now and it will erase your cache, but leave your data alone. If you're having problems, or you want to avoid problems. Erase your data in CWM before you flash the supersu zip included in the package.
For Now - Windows ONLY, Sorry.
Linux version, on the way
Download and PLEASE READ the Read Me!