Just discovered a new shortcut for the Dictionary in Mac OS X Tiger - Press
Cmd-Ctrl-D and voila, it shows the meanings of the selected word in my browser!
P.S. In case you are wondering, I'm reading the RSS feed for Sepia Mutiny.
Update: The shortcut works everywhere, not just the browser.
Update: Premshree has written a Greasemonkey script that does the same for you on Firefox, although I don't know if it's okay by OneLook's TOS since it does site-scraping.