It is located at
Simply change the "RPEnabled" entry from "1" to "0"
Changing this key changes a lot - it seems to put it back to a "Microsoft Internal" state displaying the following warnings on Winver and the desktop:
Please note, as with everything on the preview, this is a sort of "hack" which is not officially supported. It changes a heck of a lot - Pressing the start button in the bottom left corner works as expected... but, the ribbon bar disappears, you get classic Task Manager, and pressing the Windows keyboard key is buggy - the start menu appears, then disappears to reveal the metro interface, then metro interface disappears and the start menu loads!
The start menu really is not officially supported in this release and does seem to have a few bugs (mainly style related). I am not sure if Microsoft have disabled it on purpose from the preview in order to get people to use Metro and it will be there in the final product, or if it will be dropped entirely from final product - (or in the state it is in now for some sort of legacy reason.
A big downside is that apart from seeing a glimpse of the metro interface when you press the Windows key, it does actually break it and the ability to deploy/test applications on it - which is the whole point of the developer preview. So... do this once, have a look around and put it back!
(Error starting a tile application)
As you can guess, the preview is unfinished and I am sure they will change the start menu (either disable completely or simply modify behaviour) before release.
If you are not comfortable editing the registry, here are some ready made .REG files for you:
Change the key to 0 (Enable the start menu)
Change the key to 1 (Disable the start menu)
If you force close and reopen Explorer after changing this key, it stops the Metro app from flashing up when pressing the Windows Key - however - tile based apps and running from Visual Studio still do not work. That being said, you can change the target in Visual Studio to an Emulator which runs the tile applications fast. I haven't looked in to this more, but it is a really cool application that allows you to test touch amongst other things.