Hello! Just a couple of things.
First, the issue with galleries breaking XHTML validation didn't have any open tickets in WP trac so I opened one (http://core.trac.wordpress.org/ticket/10734)...the old bugs for this were focused on other things and all got closed.
Second, with this plugin enabled WordPress stops respecting the gallery setting for 'Link thumbnail to...'. I like to have my gallery thumbnails link directly to the large image file, but no matter how I set it in the entry's gallery options it links to the attachment page if this plugin is enabled.
I haven't had time to dig into the code; if I find the issue I'll let you know.