Considering Nintendo is positioning Skyward Sword as the Wii's last hurrah and has nothing to release afterwards, I'm sticking with my original guess. I really don't think they can afford to launch past June. In fact, they should probably launch earlier but they only need to launch before July 1 to be within Q2.
I assume the screen is the most expensive component of the tablet controller and I would wager it costs like $25-$30 (that may even be too much). All the other parts are relatively cheap. In raw materials, it'd probably come out to $40 tops. If Nintendo were to sell it separately, they could be nice and charge $60 and STILL turn a profit. They'd probably charge $80 to be profit serpents. This is the same Nintendo that could have launched 3DS at $180 like DSi and made about $30 and sold bajillion units.
That said, what I think is going to drive the price upwards of $349.99 are the chipsets Nintendo commissioned from IBM and AMD. Nintendo will absolutely not sell at a loss. Depending on excactly how powerful the hardware is, Nintendo could probably sell for less and literally break even or make a minimal profit (like $11 if that, just to even out the price at $350), but we all know they won't. If they can afford to launch at $299.99, they'll sell it for $50 more. The mental image I have is of Satoru Iwata licking his lips in anticipation of money.