I think the biggest launch in Japan ever was the PS2 at 600k... Why people are predicting it will more than that when there's really no big PSV game is beyond me :P
3DS had the same price and is a handheld, so it should be comparable to it. That said, 3DS had the initial "3D" hype where as the Vita is lacking a bit of that at the moment due to the immense 3DS momentum. Reports have also said that Sony have been a bit conservative with the shipment. If it does anything above 400k, then I'll be surprised







