I received 2 emails from colleagues who were hit by this bug while using the Ascom i62.
Apparently there is a BA ack issue, as noted in the bug. Its not clear if this is only impacting Ascom as the bug id references 802.11n handsets. I asked for frame captures and will post the result when I have them.
CSCud65237 Bug Details
A follow up to this blog post: There is a TID issue which is the result of a bug in the Ascom phone while using 802.11n. Cisco has modified code and Ascom is working on a fix. Another work around is to disable 802.11n on the infrastructure.