Using querystring params in the sitemap urls *should* be fine, but it has [caused problems in the past](http://www.bing.com/blogs/webmaster/f/12248/t/672649.aspx). I wonder how hard it would be to use our standard way of encoding the url instead, so the en-US sitemap would just be at /en-US/sitemap.xml instead of sitemap.xml?locale=en-US