http://support.apple.com/kb/TS3398
Immediately after updating to iOS 4, 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.
iPod touch, iPhone 3G, iPhone 4, iPhone 3GS
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 as possible.
To install the profile:
After installation, the profile will be listed under Settings > General > Profile.
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