12 Mp crop sensor is way to nowhere.
If we have a look at www.photozone.de
we can see naked figures, but this digits are very usefull.
So we can understand that 10 Mp with crop 1.5 is optimum. Higher will be worse.
The good way is also crop 1.25 or 1.3 with 10 Mp.
To buy new D300 and A700 for amateurs is silly. If you have money and love photo, buy FF, maybe even used Canon 5D for reasonable price - less 2000 USD, not
high-price crop camera. Or keep shooting by 6 and 10 Mp cameras.
It's good and enough to print A3 and 10 Mp till A2 with good lenses.

Also...
The problem will be (not visible very clear now) that additional 2 Mp will kill old film lenses and not so expensive digital lenses, photozone is the litmus piece of paper.

Or shooter must close aperture till f4 at least, because even good fast lenses will be soft and soapy wide-open.
Not all lenses, of course. But a lot of.
It looks like PC games and video-cards situation.
If you want to play in very new game, you should buy new card.
If you want to shoot wide-open and get good results, buy much expensive lenses.

FULL FRAME is the future. All lenses work. I hope that we will see Canon 5D's analogue from Nikon or Sony next year or in 2009.

One of the last article of ken rockwell give us the answer
http://www.kenrockwell.com/tech/full-frame-advantage.htm