I'm not sure how BV's store works, but for most video game chains like EB and GameStop, the number of pre-orders we get for a certain item determines how many actual copies of that item we end up getting.
For example, Metal Slug 3 for XBOX and Tales of Symphonia at the store I work at got maybe 4 pre-orders, max. As a result, we barely got enough copies of both to cover their pre-orders, and now that people decide they want it, we're lucky to get one copy a week. But even for the big games like Madden, GTA4, and Halo 2, in the first shipment I'm about 99% sure that we'll only get enough for pre-orders, and anyone who didn't pre-order will have to wait for the next shipment.
Pre-ordering guarantees you a copy, since you know that they will at least send one copy for each pre-order. I would assume that the same would be true of the DS.