Chris posted about Secure Logins, last week. In it, he describes the confusion that exists when users cross from HTTP to HTTPS via forms.
Chris isn't alone in noticing this problem. A few weeks ago, on the Security Now! podcast, Steve Gibson answered a user question about the same problem.
On Chris' blog, I mentioned that this would be a good use for Greasemonkey. (If you don't know about Greasemonkey, you should definitely check it out.)
Since then, I've had two flights, and took a little time to work out a solution to the problem.
When hovering a form button, image or submit input, you'll see a description of the form's action, and various potential problems (onsubmit, onclick, etc).
It doesn't solve all "hijacking" problems, but it's not supposed to.. it's just a quick indicator of where your form probably posts.
Even if you're not interested, take a look at the source for a cool little embedded image trick.