I've lately been having issues with some of my users and their active sync and calenders on mobile devices - Blackberries and Droids (don't have many iPhones so I don't know if there an issue with them as well). I'm not sure if it is relevant, but I did replace 2 2003 DC's with 2 2008 DC's around the same time this issue began (although we've always had issues with active sync and calendars).
Anyway, our main issue right now is that if someone gets a meeting request via email on their phone, then accepts it, it doesn't end up on their calendar.
What we have found, is the suer has to open up the calendar on the phone and accept it from there. There are also some other odd issues, like if an even changes (like a reoccurring event who's time may change) the phone doesn't get the proper update