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.
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.
Use Safari on your iOS 4 device to download the configuration profile.
After install the profile is listed under Settings > General > Profile.
 iOS 4: Exchange Mail, Contacts, or Calendars may not sync after update TS3398 [Apple]
 Topic : iOS 4 – problem with Exchange Calendar [Apple Support]
 updated iPhone to iOS4 and I cannot setup exchange sync with gmail – error says ‘Unable to verify account information’ [Gmail Support]