I'm actually really curious if anyone has yet tried this. Sadly, I don't think there are any Microsoft stores yet in Canada (and there won't be any in my area for quite some time, I'd imagine, even though my city has North America's biggest mall by many measures). I'd love to go into the competition saying "nope, if I win I'm still keeping my phone, I just love the challenge! Now, say, do you have any of those Nokia Play 360 speakers around? I hear they work really well with my N9, thanks to NFC and all that."
If you win, they want your phone in exchange for a new one.
Not sure who wins in that deal...
if you win, you keep your phone and get the laptop. if you lose, you have the option of either trading in the phone you used for the challenge for a windows phone, or not doing the trade and keeping your phone.