I happened upon this hint by accident ... I don't know how truly useful it is, but it does work..
If you use Safari's 'foldered' bookmarks in the Bookmarks Bar like I do, you know you can click on one and it'll drop down and show all the bookmarks in that folder. You can then use the keyboard to type the first couple of letters in the name of bookmark, and the selection will jump to that bookmark (or the next closest name to that first letter, just like the Finder behaves for files), then hit Return and it'll open that link in the same window.
[robg adds: I couldn't figure out how to make a bookmark selected in this manner open in a new window or tab; I don't think it's possible. I tried every combo of shift, command, control, and option, but the bookmark always opened in the current tab.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040507220919769