Email validation checking in iPhone programming can be done using RegexKitLite library (iOS2.0),
I think it largely depends on the minimum version of iOS an App will support, and if there's a built-in class that'll do the job, use it. As you hinted, you'd use some other 3rd-party framework to support older versions in a consistent way.
Right now, I'd use
NSRegularExpression, both are supported from iOS 4 onwards, which is quite likely to be the minimum supported version for new iOS Apps, if not iOS 5.