Honestly... I highly doubt that this letter is real - there are inconsistences and errors that would not be present in a REAL legal document.
First off - spelling and grammatical errors - letters like this are based off the same general base in order for them to be a lot easier to write-up and manage - only specific details (ie, name of infringer, company infringed, content infringed) would be left out, the rest would be part of a pre-written base that is used over and over again. Meaning, there would be no grammatical errors because, for one, it would have been checked over countless times and, two, it is a legal document, any errors could technically make it void.
This brings me to my second point - due to it having come from a generic base letter, it would not take them long to send them out to other sites that are infringing. Also, even at that, if all sites were getting done for the same thing, they'd send them all out together - doing them in a list, ie, sending it to you first, is complete bull for two reasons: one; it completely nullifies objectivity and basically makes it seem as though they're picking on you more cos you're first, which they wouldn't do because it is illegal and would be a kind-of stupid mistake for lawyers of all people to make and, two; it would give you time to warn other sites which is another completely stupid and amatuerish thing for a lawyer to do.
Also, why the hell would they even send it to you first of all the sites? I'm sorry to say but Serebii has way more pics and info than you and, to take it even further, YouTube is swarming in videos of Black and White - if, and only if, they were gonna deal with it in some kind of order, they'd deal with the biggest infringers first.
"Very truly yours," - does that even look like something a lawyer would even put on a letter? Hell no! "Regards" is about all your gonna get. "Very truly yours" is what you write to your partner, wife, whatever, etc.
And, another thing, legally they cannot give you 24hours to do anything - that is not a reasonable amount of time - the minimum, if memory serves, is 7 days and then they take it further. Giving you until tomorrow is a load of bullcrud and illegal - another mistake a lawyer wouldn't make.
Oh, and more - why now? Why with Black and White? Why not when you and everywhere else covered Diamond and Pearl? Platinum? HeartGold and SoulSilver? Battle Revolution? Mystery Dungeons? Any of the Ranger games? Or, hell, any website that has ever covered any kind of new release? Why now and why only you thus far? Why has there not been any other instances of websites being forced to take down content like this?
Oh, yeah, more - "any screenshots, videos, or other content that infringes Nintendo's rights in the Pokémon series or any other game."
Why only the games? Why not infringement of copyright for the movies? anime? Cards? Anything?
Oh, and even more - why from the American branch of Nintendo? Shouldn't this come under Japanese jurisdiction? Technically, it should because, for all intents and purposes, it's got nothing to do with the American branch.
To be honest, I could go on and on and on about this all night - there is so much crud in that letter it is actually laughable. Either you've been had or you've seriously bullshitted your followers and, in all honesty, I'm leaning towards the latter wherein you're a big faker.
And, on that note, if it is discovered that this letter has indeed been falsified- by you or anyone else - the person that did it will be reported and charged.