While many debate this and have a lot of good points, the evidence seems to point to the following best practices for placement of Cancel and Submit Buttons in web forms...
1. Buttons should be separated by a good distance, preferably close to each side of fixed-width or fixed-maximum-width window. Otherwise, users don't really have a clear opinion on which button should be which.
2. In the case of buttons that are each aligned to opposite sides of the window, 3/4 users surveyed below felt that the "Submit" or "Ok" button should go on the right and that the "Cancel" button should go on the left.