The problem I have seen with using "float" for multi-columnar layouts is you can get one column to resize, but the other does not resize with it.

In Michael's example, when you resize the window in IE or Firefox, the silver background behind column 2 fills in underneath column one.

Workaround: Use a red colored left border (instead of padding) on column 2 the same width as column 1, and you will have 2 colums that resize in unison.

Code change:
div#two {
width: auto;
padding: 0px 10px 5px 10px;
border-left: 150px solid red;
margin: 0px;
background-color: silver;}