It seems that the WebKit team just raised their bet in the JS Engine poker game.
Without comparing with other engines, which is a game in itself, the speed up from previous versions of Safari to the current Webkit is nothing short of amazing.
I was looking around for a Safari version 2.x to see how it compares, but I don't have one anymore. Still, a ten-fold increase between the 3.0 release and this one, impressive.
Update: By the way, I'm a Safari user, so comparing to other engines is not something that I really care, I'm much more interested on comparisons against previous Safari versions. Still, you can find another comparison between engines here.
Although other will be more qualified to answer this, I wonder how fair is comparing Squirrelfish Extreme with Google V8 on a Mac? The reason is this: I assume that the V8 running on Windows uses a lot of optimizations that might or might not be in the Mac version.