Converting iPhone app to a Universal app in Xcode 4

Solution 1:

The accepted answer is wrong. Creating a new target is how you create a separate iPad app from an iPhone app (i.e. a project that can be compiled into either an iPad or iPhone app), it is not how you convert an iPhone app to a single universal app that will run on iPhone or iPad.

All you actually need to do is switch the Device dropdown in the target summary tab from iPhone to Universal:

Thanks to ObjectiveFlash for his comment which originally provided this solution.

Solution 2:

{ Outdated / incorrect information removed. Moderators, I've asked to at least unmark my answer as accepted. Please clean this up. }

See Nick Lockwood's answer!