Great plugin. I've used it with a lot of my sites.
But I recently came across an issue with grouping image, specifically with the plugin's use of the rel attribute instead of relying on a class.
My problem was that the plugin is using rel="fancybox" to fire Fancybox, which was over-writing the rel="group1", rel="group2", etc. in my image links and breaking the groups.
In future updates, can you substitute "class" for your "rel" attribute? or is there a reason you decided to go with rel?