I've actually wondered this myself but I doubt that it will have any noticeable lasting effect on the screen. I think LCD screens are pretty safe when it comes to burn ins. You might notice image persistence which can be cleared by displaying a white screen for 15-20 minutes if it does get that bad.
On my v1 and my 3G sometimes i would forget to lock my phone when i went to sleep and would sleep for 10-12 hours if not longer with no burn ins (Drunk sleep). I wouldn't worry about it it's not like a old plasma. Lcd dosent have thoes burn in problems like the old plasmas do.