I am implementing a design that uses custom styled submit-buttons. They are quite simply light grey buttons with a slightly darker outer border:
input.button
I've found an answer that works for me on another forum. It removes the unwanted black border in ie6 and ie7. It's probable that some/many of you have not positioned your input="submit" in form tags. Don't overlook this. It worked for me after trying everything else.
If you are using a submit button, make sure it is within a form and not just a fieldset: