The packed version of the current tablesorter plugin (from http://tablesorter.com/docs/#Download) is causing a JS error on IE6. I haven't had a chance to try this in IE7, but FF is fine.
The error says something about an illegal character on line 2. Using the Minified version, or the unpacked version doesn't have this problem. (I tried to download a few times, just in case it was a glitch in the download). This one took me a while to track down - I was looking though my own code (which is much more likely to have bugs.. :). HTH. Shawn
