Well, here we are several months/releases since the original post and apparently the issue has not yet been resolved. I JUST got off the phone with Apple support. I provided info on how my 3 iPhone 5's and one iPad 2 will NO LONGER work on my Airport Express 5th Gen using 802.11n only 5GHz--however, my two MacBook Pros and two iMacs (one 2007 and the other 2012 model) work just fine on the same 802.11n-only, 5GHz network/radio. The second radio on the same Airport Express (configured as 802.11b/g @2.4GHz) works fine with all my devices.
I didn't really notice the issue with the iPhone/iPad (IOS) devices until the version before 6.1.2 (not sure what that was), but if my IOS device (my iPhone) was connected to my 802.11n network, an app update from the App store would freeze the update process while only partially complete, and would not complete (no matter how long I waited) until I changed the device's network connection over to the other (802.11b/g) connection where the active app updates would resume/complete.
Apple support has taken all this info (noting that all involved devices are at the latest software/firmware and that I'm running everything at the recommended settings) and has added to the active case of "IOS and WiFi issues". I'm crossing my fingers that they will resolve this ASAP...especially since there is NO option to roll back to a previous version of IOS (I asked, and support told me there was no way to do it).
C'mon Apple! ...don't let us down!! Get this resolved quickly please!