Apple have released an update for the iPhone IOS4. This problem causes Exchange Mail, Contactas, or Calendars not to sync, or to sync very slowly. Users might also experience increased data usage, which may result in some hefty data charges.
The Apple patch/workaround can be downloaded here – http://km.support.apple.com/library/APPLE/APPLECARE_ALLGEOS/TS3398/DefaultEASTaskTimeout.mobileconfig
The easiest way is to open the link in Safari on the iPhone and run it.
The Apple support article can be found here – http://support.apple.com/kb/TS3398