We all long for the day when <div> tags and basic CSS styling will be supported by all the major email clients, but alas the time is not now. At this point, things like position or float aren't supported by outlook or lotus notes, so that pretty much nullifies using divs for structure at all. You can still use them within table cells for universal styling purposes (or to use them in place of <p> or <br> tags) but it's not a good idea to use them for anything else, unless you have a one-column layout.
** EDIT: Campaign Monitor keeps a CSS compatibility chart that is extremely helpful http://www.campaignmonitor.com/css/ **
