Looks like JA Purity and SIG doesn't play together well.  Did a search on this forum and saw two others having issues.  On pages with SIG, looks like mootools loads twice giving a little error at the bottom left corner of IE.  Mozilla and Safari shows no notification of error.  IE 7.0 it continues to have full functionality of the JA Purity drop down menus (suckerfish) but in IE 6.0 the drop down menus go too transparent.

I tried both the free version and the pro version and problems persist.  When I switch to another template there is no problem.  Not sure if you guys can do anything about it but just wanted to make you aware since it's a popular template that joomla.org even has a little area dedicated to this template in their support forum since it won their contest. 

Anyhow, I love this plugin so I guess I'll just have to live with it for now in my galleries.