It's becoming almost a ridiculous sad joke. Each new iOS release seems to bring a new round of calendaring bugs that cause havoc with Exchange and Activesync, and create no end of headaches for users and admins.
The latest example - the iOS 6.1, 6.1.1 fiasco - has taken it to a new extreme.
Apple needs to stop dripping with hubris about this stuff, and actually test their products consistently and properly. The post-Steve Jobs downward trend is sad, and concerning. One product after another is increasingly flawed.
Anyway </rant mode>
In the mean time - read about the iOS 6.1 mess here, then immediately go and update all your i-Devices to 6.1.2. Then wait for 6.1.3 or 6.2 or whatever is going to fix the lock screen bug.
iOS 6.1: Excess Exchange activity after accepting an exception to recurring calendar event