....does not work with the, er, iGoogle mail plugin.
If you set it to "always on" (and the default here is "on when possible") all you get in iGoogle is a message saying that you need to turn it off so that your mail can be displayed. Actually, I have found it has something of a habit of occasionally coming up with that even when fallback is allowed. A close and reopen of the browser session fixes that one.
Just maybe the reason that Gmail's HTTPS function *seems* to work transparently with everything is that the default setting is to fall back to HTTP when the requestor cannot handle HTTPS?