No padlock on checkout page

-TangleFoot-

Another possible explanation for the absence of the padlock symbol could be that the secure website you are browsing uses frames, in which case the browser may actually be revealing the security properties of a page other than the one you're typing your card details into.

From .net magazine:

You can be sure you/re connected to a secure server when a) the address begins with https: b) a locked padlock appears in the Status bar and c) right-clicking on the page and selecting Properties tells you the connection is secure. Item c) is the acid test on sites with frames.