How to create a pseudo-guest account in Lion with FileVault 2?
Guest Account

This account is visible in Directory Utility, but please do not attempt to make it compatible with FileVault 2. Not only for security; there may be other good reasons for Apple making this account not available whilst FileVault 2 is enabled. 

Other accounts

For your guest you might like to create a standard account, or add parental controls. You can probably have an account that is password-free. This is in line with Kyle Cronin's answer to a linked question. 

Recommended reading

Apart from being able to encypt an entire volume, what are the other differences of FileVault 2 over FileVault?