Apple has posted a knowledge base article support note for users whose Exchange ActiveSync mail, contacts, and calendars are no longer syncing post-iOS 4 update.
Here are the symptoms as described by Apple:
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.
And the solution:
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.
If you've been having Exchange sync issues under iOS 4, let us know if this does indeed resolve it for you.
[support.apple.com, thanks Doug!]