I get the error:
"Missing the call to wp_head() in your theme, contact the theme author"
When running the nggallery compatibility checker however the gallery appears to be working fine and the theme (a minor mod of twentyeleven) definitely has wp_head() right before /head in the header.php theme file.
Just in case anyone else is getting this error and is wondering. So, if you're having problems and see this error it may not be what is causing things you can check in the Appearance > Editor in your header file (or equivalent) to be sure.