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