The letter is quite long and they apologized for posting it in the NEWS section since it doesn't really falls into the "news" category, but it was the most suitable place to publish it.
I'm gonna try to quickly summarize it.
Basically CLAMP always liked Vocaloid and the original video creations that are posted daily on NicoNico. They like to check on the videos during intervals between work.
One day one of the members found the song Mugen no Yami[original] (it did not have the suffix echo of the past yet) on NicoNico and all 4 immediately fell in love with it. But one thing bugged them: the screen was pitch black. They didn't think that was "fair" for such a beautiful tune.
For their surprise, the composer of that song was was Shinji Hosoe, who has composed music for several games of ARIKA, the game developer company where CLAMP's long time friend Ichirou Mihara works at.
Basically they talked to each other and CLAMP asked Shinji Hosoe's permission to make a video (their idea was to make a PV out of it) of the song, which he agreed. Ichirou Mihara was then in charge of animating it.
It took one month to get the work concluded, using their own share of sleep time.
At the end CLAMP thanks everyone who watched the video, commented and added it to their favorites.
Of course, this is just a quick summary, the original letter is much bigger and very detailed. So if you are interested in the full story make sure to check it (and if you want to contribute with more information, you are welcomed!). If you find any mistakes in my amateur translation please let me know!
They also added a new illustration featuring Chibi Miku: