I don't think that Nintendo will offer 3rd party support for USB hard drives. It makes sense for them to open up certain avenues for accessories, but things like HDDs are big ticket items that N wants to get all the profit from.
Beyond that, we're not going to have any need for an external HDD until we get a host of online-capable games and downloadable content (be them demos, 3rd party d/l games, whatever). So N will put a clamp down on this as long as they can if for no other reason than to keep one more potential tool out of the hands of pirates.
I think we'll see an external HDD, but my guess is that it'll only be available through Nintendo (see the XBox HDD) and it'll only come after we see many people complaining that they've maxed out their internal memory.
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