It appears that one of the unnamed “bug fixes” included in iOS 4.0.1 is the elimination of some Microsoft Exchange issues, particularly an issue in which Mail stalls on “Checking for Mail,” or cannot receive or send mail.
Apple’s Knowledge Base article on the subject now states:
“Immediately after updating to iOS 4.0, 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: Update your device to iOS 4.0.1. If you previously installed the configuration profile in this article, remove it by tapping Settings > General > Profile > EAS Default Task Timeout > Remove. Contact your IT Administrator if you are unable to remove the configuration profile yourself.”