There's actually a whole lot of significance in being endorsed by a respectable institution. As several other people have remarked in this thread, if the feds decide to throw the book at you, you're screwed no matter what. The only way to immunize yourself against this threat is to have backing by high-status people who are able to sway the public opinion and the legal establishment in your favor (so the feds will get bad press instead of accolades if they attack you, and you have a good chance to persuade a court to order the feds to leave you alone). EFF is far from being a decisively powerful player in this regard, but getting its endorsement is definitely a large step in the direction favorable for the Bitcoin people.
Again, I agree there are benefits, I just dispute their characterization by Wei_Dai et al. EFF is not "endorsing" Bitcoin in the sense usually meant; they're saying they'll accept donations that way. There's a huge difference between that and "Oh, but these respectable lawyer guys told this big organization it'd be okay!"
Anyway, I did join up. If you look at the map of users, I'm the singular dude in Waco.
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Currently Bitcoin mining appears to be profitable as only bubble economics can be. Already some Less Wrong users have purchased GPUs and started mining Bitcoin. Please consider sending some Bitcoins to SI at address 1HUrNJfVFwQkbuMXwiPxSQcpyr3ktn1wc9
2014 Edit: Please donate Bitcoin by using the Bitpay link on MIRI's donate page. Thanks!