Community Server 2008.5 SP1 contains a bug in its creation of sitemaps. This bug has been brought up quite a bit on the Community Server forums and Telligent is aware of it. They have fixed this issue internally, but (last I heard) the fix will only be released with Community Server 2009.
So in the meantime, you can download the Community Server 2008.5 SP1 SDK, make the following changes, recompile, and your sitemaps should function properly.
CommunityServer.Components\Components\CSContext.cs (added ", true")
Line 734: _applicationType = (ApplicationType)Enum.Parse(typeof(ApplicationType), QueryString[CommonQueryStringProperties.ApplicationType], true);
CommunityServer.Blogs\Components\WeblogSiteMapIndexBuilder.cs (changed "blog" to "weblog")
Line 24: string url = UrlBuilder.FormatUrl("siteMap", "weblog", "app", content.ApplicationKey);
CommunityServer.MediaGalleries\Components\MediaGallerySiteMapIndexBuilder.cs (changed "media" to "mediagallery")
Line 25: String url = UrlBuilder.FormatUrl("siteMap", "mediagallery", "app", gallery.ApplicationKey);
CommunityServer.Web\sitemap.ashx.cs (changed "blog" to "weblog" and "media" to "mediagallery")
Line 74: case "weblog": return GetWeblogWithAccessCheck(context);
Line 75: case "mediagallery": return GetMediaGalleryWithAccessCheck(context);