3.6 IE Stepdown Bug
Floated objects normally line up nicely horizontally until they break, that is, run out of width and then spill down to the next available space. IE6 has to be awkward. It adds a line break effect after each floated block element, causing stepdown. This can completely break page layout.
Fixes
Luckily the fix is simple and the same as for the double margin float bug: simply add display:inline to the floated element (another method is to set line-height to zero):
#leftcol {display: inline; float: left; width: 600px; margin-left: 200px;}