How can I boot to Safari Only (Chromebook style) in Lion?

How can I boot safari only in Mac OSX Lion Chrome OS style? I heard that there was a key that one had to hold at startup, which one is it?

I finally found the answer to this. There are two types of Guest User mode in Lion:

a) Safari-only mode: This is the default setting if no Guest account is enabled. Therefore to turn this on, go to System Preferences / Users and Groups / Guest User, then uncheck "Allow guests to log in to this account".

• Note: Under "Login Options", "Display login window as:" must be set to "List of Users" or this will not work.

b) Snow Leopard-like Guest Account - a full guest account as in previous versions of OS X. This mode is enabled when "Allow guests to log in to this account" is checked.

It appears the Safari-only mode was not included in the retail build of OS X Lion.