Looks good...just fixed a couple grammar issues and added a little flare.  I'll also add 5 BTC to this bounty, payable when/if Schneier would like to redeem the offering.

Dear Mr. Schneier,

We're writing as a community of enthusiasts and creators of an open source cryptographic currency project called Bitcoin.  Bitcoin is a decentralized peer-to-peer money system that relies on cryptography instead of central banks to enforce the integrity of transactions.  Started in 2009 by Satoshi Nakamoto, Bitcoin is being billed as "the world's first cryptocurrency," and has recently started to garner significant interest.  Though officially still in beta, Bitcoin already has nodes, participants, and exchangers located around the world.

The reason why we are writing is, first and foremost, we wanted to see if you have heard of it.  If you have not, we'd like to invite you to become familiar with it.  Second, we are eager to hear any opinion or critique you might have to share about it.

We understand your time is valuable.  We hope that after a brief look you will find Bitcoin to be truly novel, peculiar, and worthy of your interest.  Several of us who are familiar with you and your work have expressed a willingness to collect a small but significant offering if you're interested - naturally, payable in Bitcoins.

I have included a brief synopsis of Bitcoin, as well as a summary of Nakamoto's Bitcoin Whitepaper for your convenience.  Our website is bitcoin.org, and we are available to answer any questions you may have.

Sincerely, ...