From what I understand from the W3C classifications is that <table>s are meant for displaying data 'only'.
Based on that I found it a lot easier to create a <div> with the backgrounds and all that and have a table with data floating over it using position: absolute; and background: transparent;...
It works on Chrome, Internet Explorer (6 and later) and Mozilla Firefox (2 and later).
Margins are used (or meant anyways) to create a spacer between container elements, like <table>, <div> and <form>, not <tr>, <td>, <span> or <input>. Using it for anything other than container elements will keep you busy adjusting your website for future browser updates.
