Maybe I'm a straight thinker but I'm a bit puzzled, don't you know the column number of your table?
By the way IE6 doesn't honor the colspan="0", with or without a colgroup defined. I tried also to use thead and th to generate the groups of columns but the browser doesn't recognlise the form colspan="0".
I've tried with Firefox 3.0 on windows and linux and it works only with a strict doctype.
You can check a test on several bowser at
http://browsershots.org/http://hsivonen.iki.fi/test/wa10/tables/colspan-0.html
I found the test page here http://hsivonen.iki.fi/test/wa10/tables/colspan-0.html
Edit: Please copy and paste the link, the formatting won't accept the double protocol parts in the link (or I am not so smart to correctly format it).