[dgatwood]

I'm pretty sure Nook handles margins correctly (for values other than "auto", which, like all ADE-based readers, Nook handles in a horrible way that is technically compliant with the EPUB spec, while at at the same time being thoroughly non-compliant with both the letter and the spirit of the CSS spec on which EPUB is ostensibly based).

Note, however, that other ADE-based readers collapse element margins into the page margin, which can cause problems, so for a more general solution, padding is probably a better choice.