The new iOS 6.1 Beta 5 released yesterday contains a reference suggesting that Apple may be looking to offer 128 GB iOS devices in upcoming generations From MacRumors:

The reference to a 128 GB capacity comes as part of SystemPartitionPadding key in the build manifest associated with the release and is not found in publicly available versions of iOS. While the observation does not mean that a 128 GB iOS device is definitely coming later this year, this is not the first time in recent months that Apple has hinted at future 128 GB iOS devices.

 iOS 6.1 Beta 5 Code Hints at Upcoming 128 GB Devices