As per the updates from various sources, the iOS 6.1 version creates issues with Exchange Server, as it overloads the Exchange Server 2010. According to the latest updates, apple and Microsoft are currently working towards an update to workaround this issue.
This issue looks to have some relation with earlier reported meeting hijacking, as it has turned out to be happening when Apple users process their calendar updates using their iPAD or iPhone.
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Request the users not to update the new version until Apple and MS are coming up with a fix.
Update: (14- Feb-2013)
Apple has acknowledge the bug, iOS 6.1: Excess Exchange activity after accepting an exception to recurring calendar event
When you respond to an exception to a recurring calendar event with a Microsoft Exchange account on a device running iOS 6.1, the device may begin to generate excessive communication with Microsoft Exchange Server, they also have suggested a workaround,
Apple has identified a fix and will make it available in an upcoming software update. In the meantime, you can avoid this bug by not responding to an exception to a recurring event on your iOS device. If you do experience the symptoms described above, disable then reenable the Exchange calendar on your iOS device using the steps below.
Hope this helps.
Update: (19 - Feb - 2013)
Fixes an Exchange calendar bug that could result in increased network activity