Apple released iOS 4 recently and since its release there have been numerous reports of a number of issues with new iPhone 4s when using Exchange ActiveSync . Here is the link from Microsoft Exchange Team Blog

PATCH Released from Apple : iPhone users who use the iPhone4 or upgraded to iOS4 must install a related Apple configuration patch on their iPhone. The patch, along with installation directions, can be found here:
http://support.apple.com/kb/TS3398.