I am running 7921's with PEAP authentication back to ACS 4.2 but often get "Authen session timed out: Challenge not provided by client" failed authentications in ACS. Leading up to the failure, I see the phone authenticate about 2 minutes before in the most recent case. Sometimes there will only be one failure and sometimes 4-5 in a minute.
I'm thinking about switching authentication to EAP-FAST but curious if others are using PEAP and running into the same thing.