I did that twice with both my white and my wife's black on two separate occasions at our King of Prussia Apple store. Grabbed an associate, pointed to a picture of the mid-range configuration and said "I want one of those."
She went back, came out with a gorgeous box, and I paid for it on the spot.
When I went back a week later and did similarly, the only question was "okay, what color?" A minute later I was paying for a black model.
So if you have a store within travel distance, and don't request any upgrades (which you would save on third party do-it-yourself anyway) yes, you should be able to walk in credit card blazing and go home with your new companion.
Heck, my brother-in-law ordered one from the online store and got it next day from FedEx.