Got this email from our mail guys after upgrading to iOS 4.
Not sure if the issue is with upgraded 3G/S only, or also with iP4.
We are alerting all iPhone users connecting to our corporate mail servers to advise you of a major issue regarding the iPhone OS4 update. Certain users that have upgraded to 4.0, upon their initial calendar sync to our production mail servers, are causing a large back end impact on performance for all users of their mail server. The issue has been reported to Microsoft and they are reporting additional customers experiencing this same impact. While we are actively working with our vendors for a resolution please refrain from upgrading your iPhone to 4.0 until advised by IT to do so.
Not sure if the issue is with upgraded 3G/S only, or also with iP4.