When an iPhone app is updated and the minimum required iOS increases, what happens for customers who still have the lower iOS? For example, I'm a developer, and my app is released with a minimum iOS of 3.0 required.  If I release an update requiring iOS 5.0, what happens?
Can existing customers running 3.0 attempt to update?  What error message (if any) do they receive when attempting this?
Will customers running 3.0 get the 'Update' notification in their app store app?
When the user tries to install the update, the app store will alert the user saying something like "you must have iOS5 to install this update.". The user will still be able to use the current version they they have, though.