SymptomsImmediately after updating to iOS 4.0, some users may notice that Exchange ActiveSync Mail, Contacts, or Calendars do not sync, or sync very slowly. In addition, some Exchange Server administrators may notice their servers running slowly.Products AffectediPod touch, iPhone 4, iPhone 3G, iPhone 3GSResolutionUpdate your device to iOS 4.0.1 or later. If you previously installed the configuration profile in this article, remove it by tapping Settings > General > Profile > EAS Default Task Timeout > Remove. Contact your IT Administrator if you are unable to remove the configuration profile yourself.If iOS 4.0.1 or later is not available for your device, proceed with the following:To work around this issue, users should install a configuration profile from Apple that increases the amount of time the iOS 4 device will wait for the Exchange Server to respond to its sync requests. For best results, the profile should be installed on as many iOS 4 devices at your institution as possible.

via iOS 4.0: Exchange Mail, Contacts, or Calendars may not sync after update.