Randall Munroe (xkcd.com). One of my earliest bitcoin-related thoughts was that he is going to make a fortune as soon as he puts up a bitcoin receiving address.
[Edit: looks like Randall is way ahead of me: http://xkcd.com/bitcoin/ -- too bad]
Wil Wheaton (Westley from ST:TNG). Respected (yes, really), huge geek fanboy following (e.g. on Twitter).
Joss Whedon (creator of Firefly, Buffy The Vampire Slayer, others). If he puts bitcoin into one of his stories, the fight is over. Game, set, match.
We should send them letters... Lots of letters.
joss whedon - yes. those sci-fi guys are always looking for something new to call the currency used in the worlds they create.
also... what about Serj Tankian?