I have noticed this for some time now. For some reason only the product_cat page in the XML sitemap does NOT work and google webmaster tools show this as an HTML error.
Also, although I think this is unrelated, all urls in sitemap give the "Sitemap contains urls which are blocked by robots.txt" in webmaster tools.
WooCommerce version 220.127.116.11
WordPress version WP 3.4.2
WordPress SEO by Joost de Valk version 18.104.22.168