Apple has posted an iOS 4 Exchange Server sync fix (http://support.apple.com/kb/TS3398). Immediately after updating to iOS 4, some users may notice that Exchange ActiveSync Mail, Contacts, or Calendars don’t sync, or sync very slowly. In addition, some Exchange Server administrators may notice their servers running slowly.
Apple says 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 company or home as possible.