Have you looked at the Python based one provided by google that is installed at the server level?
It's pretty intensive, but auto creates multiple sitemaps based on a defined # of url's per sitemap... at which point you simply need a single sitemapindex file to point to all your sitemaps.
See reference here (also note the links on the left hand menu for installation, configuration, etc:
http://googlesitemapgenerator.google...reference.html
Downloads are here:
http://code.google.com/p/googlesitemapgenerator/