Gmail HTTPS.....
....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?