I know Google's webmasters downloads a new version of my sitemaps every day. I am not sure if the yahoo / bing systems do the same thing. Google's says "Downloaded" and for the sitemaps that add the daily content they are downloaded on a daily basis. But the Last Submitted date on Yahoo site explorer is just the date of when I originally linked them into yahoo.
There is nothing I see to suggest Yahoo is rechecking my submitted sitemaps, particularly since my page index count seems to go down.