<p>Is it acceptable to submit from an http form through https?  It seems like it should be secure, but it allows for a man in the middle attack (<a href="http://ask.metafilter.com/48531/are-http-forms-posted-thru-https-secure" rel="noreferrer">here is a good discussion</a>).  There are sites like <a href="http://www.mint.com" rel="noreferrer">mint.com</a> that allow you to sign-in from an http page but does an https post.  In my site, the request is to have an http landing page but be able to login securely.  Is it not worth the possible security risk and should I just make all users go to a secure page to login (or make the landing page secure)?</p>

Posting a form from an http page to an https page does encrypt the data in the form when it is transmitted in the most simple terms. If there is a man-in-the-middle attack, the browser will warn you.
However, if the original http form was subjected to man-in-the-middle and the https post-back address was modified by the attacker, then you will get no warning. The data will still actually be encrypted, but the man-in-the-middle attacker would be able to decrypt (since he sent you the key in the first place) and read the data.
Also, if the form is sending things back through other means (scripted connections) there may be a possibility of unencrypted data being sent over the wire before the form is posted (although any good website would never do this with any kind of sensitive data).