Update: iOS 6.1.2 resolves this problem
I have see a lot of reports lately about the Apple iOS 6.1 bug effecting organisations using Exchange 2010. The problem appears to occur when a user using a Apple device (iPad, iPhone, etc) with iOS 6.1 responds to or edits a single instance of a recurring calendar event, this causes excessive log growth and a significant increase in memory and CPU resource consumption in turn causing server performance to be affected. Both Microsoft and Apple have released support articles for this bug, but at the time of writing it has not been fixed.
If you do currently have a large iOS device fleet, I would suggest advising your users against upgrading their devices until this is resolved. If you are already experiencing resource problems on your Exchange servers, I strongly suggest using either a custom throttling policy for iOS 6.1 users or blocking iOS 6.1 users completely. For more information on these workarounds, see the following Microsoft support article.