There was also a quiz for just this sort of thing on Sitepoint, for those looking for a non-JS solution. However I found that the table footer was necessary for stopping the table headers from collapsing their widths IF the contents of the cells weren't wide enough. I ended up hiding the tfoot in the application I used this on.
It's pure HTML/CSS and works in IE6 plus modern browsers. There are some styling limitations for the header though.
