Did anyone else spot Safari 4.0 on Steve's page? Is that some Apple folks trying out their latest offering, perhaps? I'm pretty sure WebKit nightly comes out as WebKit.
Unless you have installed Safari 4 Developer Preview (available to any body with an ADC account), your WebKit.app User-Agent string will say Safari 3.x, with a nightly revision number. The exact same revision on a system with Safari 4 will have a different string, though. So it's entirely possible to have a newer version of WebKit running in an older version of Safari, and vice versa.
I think any nightlies since the DP was released in August _ought_ to be counted as Safari 4, but some may not be for this reason.