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 had this problem and solved it with a div around the button, displayed it as a block, and positioned it manually. the margins for buttons in IE and FF was just too unpredictable and there was no way for them both to be happy. My submit button had to be perfectly lined up against the input, so it just wouldnt work without positioning the items as blocks.