This is odd, and wonder if anyone else has noticed this. Usually my sitemap is downloaded at least daily. It hasn't been downloaded by G in over 2 days. I was also included in the Yahoo directory last week and G isn't showing that link yet when I do a search (not a backlink search, just a regular search for my URL). Usually my new links show up faster than that. Something to do with BigDaddy nearing completion? I think somethings up at G!
When sitemaps started it would check witin hours. But now it is not uncommon if it checks once evey 2 or 3 days.
If you update pages you should resubmit the sitemap. Sometimes it will recheck itself but better to resubmit it to let google know it has changed. Apologies if you already know this and I just am misunderstanding you
If you use the ping-command (which causes a resubmit) it will download your sitemap within minutes (at least at the moment, ha ha): https://www.google.com/webmasters/sitemaps/docs/en/submit.html#ping