According to PGC, a boot disc will be used. Also according to PGC, Nintendo plans to release various colors in the US too. And about your question refering to just buying a US boot disc when it comes out here, um...that seems kinda weird, because you would have your GB player, but you wouldn't be able to play it until it comes out in the US. But, I believe theorectically this would work, because the GBA is "territory-immune" and the GB player contains GBA circuits.