can replace HD
I called the Apple online store back, and this time the guy I spoke to went and checked with his adviser, a well seasoned sales person. The user can replace their own hard drive and it does not void the warranty, unless you do some damage, which, of course, they won't take responsibility for.
That's good news for me as I plan to purchase from MacConnection so I don't have to pay any sales tax. Then I can buy a 7200 500 GB HD and put that into my 15", take the slower one that comes with the computer and put that into an enclosure and use it for an external backup HD. Makes sense to me! And saves me $200-300.
Thanks everyone!