Recently those three major search engine are teaming up so the end users can benefit with just using one sitemap for that’s based on Sitemaps namespace definition. And that means you can save more space on your hosting, because with just using one sitemap, it’ll cover everything.
Unfortunately the MSN Livesearch team are a little bit slow about supporting this format because they want to test everything first before it’ll be available for public, and as you can see Yahoo Search Engine team already upgrade their search engine so it’ll support the new sitemap format right away, and that means you can try submitting your new sitemap format into Yahoo once again.
As for those who’s already using Google Sitemap, you don’t have to do anything else other than changing the xml namespace into the correct xml namespace from Sitemaps.org documentation. And as a side note, using sitemap doesn’t guarantee your page will be on top search engine result or getting listed on search engine. But it does help search engine to crawl your site properly.