WordPress version: 3.4.1
WordPress SEO version: 1.2.5
During some routine testing I disabled and then re-enabled the WordPress SEO plugin. Afterwards, attempting to visit /sitemap_index.xml returned a 404 error.
I went to the Yoast WordPress SEO: XML Sitemaps settings page and the checkbox to enable XML site map functionality was still checked. I clicked the Save Settings button and the sitemap started working again.
I was able to repeat this by disabling and enabling the WordPress SEO plugin again.
I verified the same result on a test install running only the WordPress SEO plugin and the Twenty Eleven theme.