Not specifically iPhone 4 related, for those of you struggling with speed issues with iOS4 on the iPhone 3G, here is some relief:
http://lifehacker.com/5600825/how-ca...n-my-iphone-3g
To summarize, Apple is looking into the speed issues, and disabling (or limiting) the Spotlight Search function makes the phone usable again. I tried all sorts of restores, resets, etc. and limiting Spotlight Search to just contacts and email has really sped things up.