Over a year ago, we reported that Google Recommends You Split Sitemap Files By File Type. The issue back then, is the same issue some webmasters are experiencing. When you want to create a geo-sitemap, you have to make that sitemap it's own file. Why? Well, because it confuses Google.
The <geo:geo> <geo:format>kml</geo:format> </geo:geo> line, if found in the normal sitemap, produces an "Invalid XML tag" error. The only way to fix it is to separate out the geo-sitemap, as it's own sitemap. Having multiple sitemaps is not a problem, but it might be a little more work for some.
The thing is, it was assumed that this was a temporary requirement and that Google would allow you to add geo specific data elements to your normal sitemap. Over a year later, it is still an issue and new threads are asking why.
Forum discussion at Google Webmaster Help.