Although the code isn't perfectly semantic, I think it's more straightforward to have what I call a "clearing div" at the bottom of every container with floats in it. In fact, I've included the following style rule in my reset block for every project:
.clear
{
clear: both;
}
If you're styling for IE6 (god help you), you might want to give this rule a 0px line-height and height as well.