It appears to be fake... Ambi Pom-Pom of PokeBeach posted a good argument regarding the document. But simply assuming that the notice is invalid can only hope to land one in a world of legal trouble... :( I noticed Serebii removed BW images as a preemptive action...
IIRC, someone from the anime did an interview with someone of PokeBeach, and since Nintendo is affiliated with the anime, they have likely known about the site (as well as others) for a long time. Prerelease HGSS images were posted on PokeBeach and other fansites, and Nintendo didn't have a problem then, so apparently Nintendo must be OK with the whole thing, as long as the fansites do not make profit off such images.
And really, these fansites are helping Pokemon in a sense via "promotion" that Nintendo doesn't even have to pay for. Thousands of fans outside Japan are already stoked and set to buy the game, despite some "perceived flaws". And even still some are waiting for the third version of BW that hasn't even been ANNOUNCED yet (myself included). :D
I understand the property is Nintendo's, so they have all of the right to send out such notices. But really, as far as I see it, no "substantial damages" are going to be incurred by Nintendo if Pokemon fansites continue to propogate information that 'tides us over' until other localized releases come to fruition. None of the fansites are even trying to tarnish Nintendo's name/image; in fact, they're doing the opposite.
The only possible trouble that could arise is regarding the ROM sites, and a lot of Pokemon fansites I've seen have explicitly forbidden discussion of such items. Not to mention that most people tend to buy the actual games anyway...
And if the letter is fake, I hope whoever wrote it does get reported and punished, because it is just wasting everyone's time. I had thought Bulbanews had posted another BW article, but instead I encountered this mess...
EDIT: And I just noticed how the letter refers to Nintendo as "we" in the first paragraph, yet calls themselves "me" in the last paragraph. So that must be one of the several grammatical/typographical errors being referenced by others... :\