A Google Groups thread reports an issue (or a feature, depending on your view) with Google's Sitemap handling.
In short, a webmaster used one Sitemap file for all his content. In that file, he specified the geo data by using <geo:format> tags. When the Sitemap was produced in Google, Google returned an error that read, "Status: Invalid XML tag." This confused the webmaster, because the tag is valid.
JohnMu came in, suggesting that he split the geo content in the Sitemap into a new Sitemap file. John suggested, "that you spit your URLs into separate Sitemap files based on the kind of content they're pointing to." He explained, "make a single Sitemap file just for your geo-content, and a different Sitemap file for (X)HTML / web-search based content."
Why does Google want you to do it this way? John says, it "makes it easier for us to recognize the kind of content that you're pointing to and helps to makes sure that we route it to the appropriate group within Google."
This is something to keep in mind when creating Sitemap files.
Forum discussion at Google Groups.