I found a nice three column layout based purely on CSS here and though it would be nice to convert to a s9y theme.
From the original site:
Everything in EM (change your browser font size to see the effect)
Full height columns (or any column can be longest)
Switch between one two or three columns
Fluid center but fixed width side-columns
Min-max width for IE (experimental)
When converting it to s9y, I tried to move most of the default styles over, so it looks very much like the default theme. One clever thing was using smarty templating to select the css elements to hide the left or right sidebar divs when there were no plugins in those sidebars. Works pretty well.
I wasn't able to get the css includes working, so I just threw everything in one css file. It is still broken out into layout, presentation, and element sections, with the s9y specific stuff all at the end.
The Internet Explorer maxwidth script workaround seems to generate an error, but appears to work. hopefully someone smarter than I will be able to fix it.