I read a great article by Luke Wroblewski talking about Primary/Secondary form actions. He worked with the UK-based usability firm Etre to see which pattern of Submit/Cancel buttons work best for completion times and usability on web forms.

I agreed with the results of the study and the philosophy that while it may take a second or two longer for the user to decide between a primary and secondary button that look different, it helps eliminate incorrect actions from being taken. Changing the color of the buttons isn’t always enough because people will still take a moment to figure out which one is correct for their action, but if you make them physically look different, it helps distinguish which action is more than likely the one you want to do. This can help ease the user along.

Link to the article: Primary & Secondary Actions in Web Forms