Odd issue that I'm having trouble debugging.
Latest version of wordpress-seo, on WP3.1... the sitemap generation is creating a bad date for the toplevel site.
The "last change date" ends up being something like "3094-03-18 04:30". Each time I regenerate the sitemap, the year of the date goes up.
I can't quite figure out how the blog modification date is gathered. but I also don't see anything that changed with wordpress-seo with regards to this, so I'm not sure why it would have broken just now.