From MS Exchange Team Blog:

Recently Apple released iOS 4 (the new name for the operating system that runs on iPhones, iPod touches, and iPads). Since its release there have been numerous reports (link, link, link) of a number of issues with new iPhone 4s (and older iPhone models running the updated software version) when using Exchange ActiveSync (EAS). I wanted to put up a quick posting about what issues users may be seeing and what we’re doing about it.