We have Cisco branded 1142's and have noticed the same thing. It seems to be the combination. If we disable N radio....things work. Or, disable admission control and it will start working. iphone 4 on ios6 and iphone 5 on ios6 behave the same way. iphone 4 on ios 5.1.1 works.
Same problem here. I'm using Enterasys 802.11n access points. With admission control (TSPEC) enabled, iOS6 iPads (e.g. 3rd gen and 4th gen) I cannot view websites that contain video (e.g. YouTube). It is fine once TSPEC is disabled.