Apple has posted a configuration profile which aims to fix exchange problems noticed by customers updating to iOS 4
Symptoms 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.
Resolution 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.
Click this link from your iOS 4 device to install the profile.
You're a bit behind the curve here. As of yesterday morning, it was determined by users on the forums that the problem is directly related to the number of calendar entries a user has. The greater the number of entries (anything on your calendar older than 1/1/10 will probably put you over the "limit"), the better the chance you'll have the issue to some degree. The only real fix is to delete or archive your old calendar entries.