Now that Apple iOS 4.0.1 is available for iPhone 3G, 3GS and 4 devices, to deliver best service experience, Microsoft Online Services is "requiring users with devices running iOS 4 to update to 4.0.1 by July 29, 2010. After July 29, users who've not updated to 4.0.1 will be unable to connect to Exchange Online via Exchange ActiveSync to send and receive email and synchronize message and calendar items. This only affects users running iOS4.0.0; devices with previous versions of Apple iOS, such as iOS3, are unaffected," stated Microsoft.
"4.0.1 contains updates for several issues, including configuration patch needed for iOS4 devices to use Exchange ActiveSync with Exchange Online."
More Info: Apple support knowledgebase, Article DL1061