Anything is legitimate currency if someone is willing to accept it as such. BitCoin isn't a scam but it very well might be a bubble. It wouldn't be the first one and surely wouldn't be the last one.
If anyone is interested in the crypto behind BitCoin check out Security Now expisode 287. Audio or Video is here and the transcript is available here.
As Tim said above, if the crypto involved in BitCoin is cracked all transactional security is essentially no longer relevant. That goes for transactions involving traditional currency too. The only way that wouldn't be the case is if it was the implementation of the crypto as opposed to the crypto itself that was faulty but that is true of everything.
But I can hold a paper dollar bill in my hand and buy something at the store. I won't need transactional security. We can go back to paper log books and ledgers for bank transactions. I don't think bitcoins will survive in that way.