I bought an iPhone 4 yesterday, jailbroke and installed Winterboard on iOS 4.0.1. Working fine with one SB theme (Aqwoah), an SBSettings theme, Backgrounder badge theme, and one LockInfo theme (High Gloss). So far everything is working great.Winterboard isn't compatible with OS 4.0 yet. He said he still has a lot of work to do to make it work.
You'll have to SSH for now.