pmmenneg - 2:55 pm on Apr 9, 2010 (gmt 0)

Just to close this issue out, it looks like the link I provided is correct, once you authenticate all of the domains you have in a single sitemap, you can validate said sitemap with Google Webmaster Tools... for some reason it kept throwing errors while I was doing it right after validating the other sites, but went back in today, a week later, and lo and behold, the sitemap is now valid and is being used by GWT.

Paul