iOS 4 on iPhone and iPad and Exchange 2003 just don't seem to be playing nicely together, especially when it comes to Calendar events -- they mysteriously disappear from ActiveSync, invitation replies refuse to send, and, well, don't get me started.
Both problems are serious for enterprise users. In the first you create or accept and Exchange event invitation, it gets added to your Calendar, and then -- poof! -- it's gone. But only from ActiveSync. If you look in Outlook or OWA (web interface) it's still there. If you're relying on ActiveSync on your iPhone or iPad, you could miss your meeting, call, conference sessions, etc.