In Safari Beta 2, say you go to Google, or slashdot (or any other recently visited site in which you have been using their forms), click in the search box (or form box) and then hit the down arrow. You'll see a list of recently used entries, and you can use the down arrow to select one.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030423001829897