Sitemaps are good, but they take time too....
Breaking into subdomains works pretty well for PR but is also slow.
I'd pick 50,000 "key" pages and ping them all over a couple months time. A "slow burn" on pinging the pages tends to do very well. All 500K over the course of many months also tends to work very well if you want to get them all.
But no matter which method, take your time. Too many too quick will get you sandboxed.