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??)