Update: I am an idiot. Turns out this is the default behavior in all versions of Safari, which although I've used a mac for years, never used enough to notice this behavior (just used Firefox). I am a loser. Also, this functionality rocks.
Original, Misguided Post:I've been using WebKit nightlies as my primary browser on my Mac at home for a while now, and it works out pretty darn well.
Sometime in the last couple of revisions (yes, I do download whenever my launch page tells me to) they've changed the incremental search functionality, very much for the better. When it's on, the page you're looking at dims, and as you type, the incremental search is highlighted.
It's a little more elegant than FF 3 I think, in that it's pretty clear that you're in the incremental search mode (by dimming the window) and not in normal navigation mode, and the highlighting of the search term was very well done.
Honestly, I didn't think something so simple could make me go Wow, but in this case it really did work out quite well.