@Takayuki - It may not be a problem with Google XML Sitemaps. I'm using a different plugin for my XML sitemap -- Yoast's WordPress SEO -- but and I'm having the exact same problem.
<url>
<loc>http://www.generalkinematics.com/?post_type=wpcf7_contact_form&p=2259</loc>
<lastmod>2013-03-11T20:59:08+00:00</lastmod>
<changefreq>weekly</changefreq>
<priority>0.8</priority>
</url>@WebsiteDoctor, I didn't understand your workaround. Can you please elaborate?