My understanding (but I may be wrong) is that WebKit is only the rendering engine. It uses whatever Safari version you have installed.
So, for the Safari 4 beta Apple took the WebKit version of day X and wrapped the Safari 4 GUI around it. There are new WebKit nightly builds almost daily which in most cases only have marginally incremental benefits. But those are benefits, after all.
And, I like the WebKit icon more than the Safari icon 😀
Well, WebKit nightlies are builds that represent the current development. They may be not as stable as builds that are officially taken over by Safari or whoever...
That being said, I never experienced any issues. Except one: WebKit does not load the Saft plugin anymore... (BTW, someone got a workaround for that??)