after applying the iOS 6.1.2 update my iPhone 5 does no longer connect to my 5 GHz 802.11n wireless network at home. No problems with 2,4 GHz. My iPad 4 and iPad mini, also with 6.1.2, have no 5 GHz connectivity problems.
My router is an AVM Fritz!Box 7390 running 5 and 2,4 GHz Wi-Fi simultaneously.
Stupid me. After switching Wi-Fi in my router off and back on again the iPhone picked up the 5 GHz network w/o problems. Can't believe this.
I just did not think about the router because all my other devices (and I've got quite a few) had no problems at all, so I thought this could not be the culprit. But it obviously was.
Good luck to all the rest!