If you want Google to index your web site properly having a sitemap is a big bonus. Unfortunately a Google Sitemap is not the same thing as a ASP.NET sitemap. And that is a bit of a shame because you are quite likely to have the latter around. Well both are XML based files so converting from one to the other can’t be all that hard right? Right! and best of all you don’t even need to do it yourself because Bertrand Le Roy has already done it for us. See http://weblogs.asp.net/bleroy/archive/2005/12/02/432188.aspx for his blog post about it or download it from GotDotNet at http://www.gotdotnet.com/codegallery/codegallery.aspx?id=237330e0-65c5-4ebb-b62b-e486dd598604.
Note, all you need to do is upload the GSitemap.ashx file to your website and it will read the existing Web.sitemap just fine. All you need to do next is submit it to Google Webmaster Central at http://www.google.com/webmasters/