Lance has definitely clarified what this issue is (not hardware, but software).
given what you are telling us, it sounds like the store employees didn't know what you were talking about.
It's funny that they don't have any computers connected to a second monitor though. One can speculate but that's not why we're here.
As to the 15" with TWO display adapters, I don't believe him. He's outright lying unless he's talking about DisplayLink drivers using a usb to video out (which has horrible support for lion at this time) and or a thunderbolt display that he daisy chained a second monitor to. In no way does his MBP out of box have two display ports.
He may have been referring to the dongles.
If you could, go back to the store with your MBP and tell them you want to plug it into a thunderbolt display. Then hit the Lion Fullscreen mode and show them what you are talking about.
You could have also brought up these forums which describes the issue in detail.
lastly, there is a difference between fullscreen in lion and the fullscreen feature that is in lion. He might assume you meant hitting the green button in the window to make it stretch the size of the screen.
He also might have said, it works fine, but didn't mention the fabric background that takes up his second monitor (which is the issue here)
Me: late 08 MBP with miniDisplayPort connected via HDMI to LCD tv and also a USB to DVI adapter connected to a samsung syncmaster. (I have a similar setup, I assume, to the one person you talked to, but I do not have function in any of the other monitors as they turn into fabric once I fullscreen.
I don't know what you were trying to prove to them, they're workers, not the people who make the products. And on that note, we're users like yourself, not experts 😉
if you want to tell apple you wose about Fulscreen and Lion send feedback.
http://www.apple.com/feedback/macosx.html