First I would like to say thank You for your great SEO plugin!
I'm just making a WP3.5+WPML based site using Your must have SEO plugin. I noticed that in my secondary domain (WPML different domain setup) the sitemap_index.xml is lost, a whote page appears instead of the usual well formatted one.
I can find the XML in the HTML source, so maybe just some minor thing: http://kerekmese.hu/sitemap_index.xml
To reproduce this I used my other site (you can look at this: http://vassmonika.hu/sitemap_index.xml) where it used to be active the WPSEO v. 1.3.3. After upgrading to v. 220.127.116.11 it reproduced the missing vusiality problem.