From the Safari 4 beta release:
* Full History Search, where users search through titles, web addresses and the complete text of recently viewed pages to easily return to sites they’ve seen before;
If you’ve been a Beagle user in the last 3 years this has been supported for Firefox and Epiphany users. But I wouldn’t mind seeing Firefox have this sort of indexing and search built-in either… the AwesomeBar was a great first step in that direction.