Apple published an update for iOS4 – iOS 4.0.1 – which should fix the synchronization issues between iPhone users using iOS4 and Exchange Server.
The issue was discovered after Apple released iOS 4.0, after which many people reported issues syncing their updated iPhone 4, iPhone 3G, 3GS or iPod Touch devices with Exchange. Initially Apple released a workaround in the form of a configuration profile which increased timeout values to 240 seconds, instead of 30. Note that iOS 4.0.1 also sets the timeout to 240 seconds as the new default.
You can view the support article (ts3398) here.