Just been reading the CF-Talk list and came across this from Jim Davis for Gmail

Just create a new folder someplace (I've only done this with personal folders, but I see no reason that it wouldn't work with system folders as well) and call it "Gmail". Then right click the folder and choose "properties". Now select the "Home Page" tab. Check the "Show home page by default for this folder" and enter http://gmail.google.com into the address. Then press "OK" and you're set.