I've checked this issue out- It's all over the web. Anyone that's written a sitemap program have come across it. Even Google's Sitemapper uses this attribute (and then causes issues in Google Webmaster). I cannot find any official response from Google regarding the issue, apart from a French Google Group post where someone said it was a clash between the attribute name and the tag name <URL>.
On the plus side, while it causes a warning in Google Webmaster, it does not stop the sitemaps from being processed.
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