Though I hope you're right about it being a built in GB player, it would make just as much sense for Nintendo to make long out of print GB games available for download on the virtual console, (nothing of this sort has been confirmed, however). There's no way they'd make DS games playable on Wii, especially since wireless connectivity between the two has been confirmed, not to mention it'd be unwieldy. It's most likely that the flap is a slot for SD cards or other expandable memory, which will function as memory cards and storage for virtual console games.