For Fedora 13, running under KDE this works for me: (go to System Settings->Default Applications and select "Custom" and put the following. This assumes you are using Google Chrome for a browser)
/usr/bin/google-chrome "https://mail.google.com/mail?view=cm&tf=0&to=%t&subject=%s&body=%B"
Subject line doesn't always seem to make it, not sure why but it's closer than nothing!