Hi cwodwrth and everyone,
I have a very similar issue. I have 'custom post types' or portfolios as they are known with my theme and am trying out Yoast on a small subsite that only has 'one' portfolio and the site map I submit using WordPress SEO (1.4.6) to Google Webmaster Tools flags up this issue...
Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead.
Clearly a similar issue and I like cwodwrth would love some feedback on this :)
Many thanks