I found it! It's hidden.
Originally Posted by xzombiex66
sucks so bad that they didnt include the ability to switch launchers like on the TF300.
The setting is there, they just hid it for some reason. To get it you need a launcher like ADW, Nova, Apex, etc that allows you to add activities as shortcuts. Once you've installed one, do the following (instructions are for ADW but will give you the idea for the others).
1. Long press home screen to get context menu. Select Add>Custom Shortcut>Activities.
2. Scroll down the activities list to Settings and tap to expand. Then, scroll down to the Switch Launcher activity within the Settings sub-menu and tap to select.
3. Tap OK to create the shortcut, then tap it to launch the hidden menu.
4. There's only one option there, to the effect of "Switch to stock 4.2 launcher" or something like that. Tap it and you'll get a pop-up telling you the tablet needs to restart to apply the changes. Hit OK, wait for the reboot and enjoy the stock JB experience.
I'm trying to spread this one around so feel free to refer people to this thread for the instructions. It aggravates me that Asus felt the need to block us from having this feature. I've been using it for two days now and it works perfectly.
Attaching a photo as evidence of the fruits of my discovery.