From your description, it sounds like this is simply a scratch, rather than an actual crack in the glass. Is this correct? Assuming it is a scratch - while you may not remember doing anything to cause it, accidentally scratching the glass screen of these devices doesn't take much effort. Unless you are really really careful it is inevitable that with the passage of time and frequent use there will be more scratches.
If you are returning a product within Future Shop's post-purchase time window, that product must be in the same condition as when you purchased it. In this case, a noticeable scratch or a crack in the glass would likely be interpreted as physical damage you caused - in which case, Future Shop may refuse to take it back. However it never hurts to ask - just bring it in to the store you purchased it at, and speak with one of the customer service reps.
And finally - if you want to minimize the risk of permanently scratching the screen in future, consider buying protective film for it.
