I posted in the forums here http://forums.microsoft.com/webmaster/showpost.aspx?postid=4328144&siteid=79

It seems you can overcome this limitation by creating a sitemap index file to aggregate your site maps into one and then submit the sitemapindex file as your site map. You can name it whatver you like so in a multi site installation you can create on for each site.

I just created one for mojoportal.com like this:

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<sitemap>
<loc>http://www.mojoportal.com/SiteMap.ashx</loc>
</sitemap>
<sitemap>
<loc>http://www.mojoportal.com/Blog/BlogSiteMap.ashx</loc>
</sitemap>
<sitemap>
<loc>http://www.mojoportal.com/productsitemap.xml</loc>
</sitemap>
</sitemapindex>

Make sure you save it as utf-8 not ansi. Name it whateveryoulike.xml and put it in the root of your site. Then submit it as your site map.

Best,

Joe