I am using it right now with a couple ESRI projects and seems to work fine. Although xcode 4 does seem to hammer the performance of my machine. Definitely read the xcode docs as its a bit different.
I install the ArcGIS API for iOS in XCode4, but I can not find ArcGIS Project Templates in XCode4! Has anyone tried using Xcode 4 with the ArcGIS for iOS API? Thanks!