It might be a ‘bit of a work around, but there is a way you can get a secure login for Google’s Calendar application. If you use Google Calendar, I would suggest you give this a shot for more security.
In fact, even if you change your Google Calendar bookmark URL to https, you’ll often be redirected to a plain old http connection. boo, Google! That’s why I’ve found Mark Pilgrim’s GMail Secure Greasemonkey script invaluable. It’s built to force a secure connection for GMail, but the script itself is so generic that it works for other sites.