This really should not be a surprise to anyone. If Bitcoin is not already illegal (and I have reason to believe it is due to most states' money transmitter laws), it's almost guaranteed that it will be soon.
If this is illegal, then I think there's a freedom of speech issue here given that essentially the protocol is just sending messages (with no copyright infringement or anything like that) to peers... Edit: of course, you could say the same about other digital transmission of currency, but it seems different due to bitcoin not being tied directly to any "real currency". Then again, IANAL.
With this message, I hereby transfer to jnhnum the sum of $12.54 out of my checking account.
(Translation, since that's probably a bit subtle to just throw out there: In the process of abstracting money transfers as "just sending messages" your abstraction has leaked critically important details. There's more to money transfers than "just messages", and it is precisely those differences that are why you will not be finding $12.54 from my bank account in your account as a result of this "message" that I have just sent. Your abstraction is not a good one for any purpose, practical or rhetorical.)