I just bought my new MBP yesterday and I swaped the hard drive with my own hard drive everything works but how would I get the new 10.8 os once its released?
When you sign into the app store it will read your serial number and ML will display "download" instead of "purchase". Sometimes it won't display that until you go to purchase it then it will tell you that it is a free download for your machine. This is based on Lion and iLife apps for the 2011 MBA that I had.
Apple computers are serialized, they can tell by just using the serial of your machine. If you're that concerned, you can verify it in your About This Mac info.