WordPress version: 3.4.1
WordPress SEO version: 1.2.5
WP Minify: 1.2.0
On finding sitemaps not working as expected I have tracked it down to if WP Minify is active the post sitemap returns:
XML Parsing Error: no element found
Line Number 1, Column 1:
page-sitemap.xml and pn_directory-sitemap.xml work as expected from:
I went back to having the default Twenty Eleven theme with only WordPress SEO and WP Minify active.