Zerocoin?
https://en.bitcoin.it/wiki/Zerocoin--
From the BitcoinMagazine article: "Some of the proposals which have been suggested in the past include: adding the capability to reverse transactions, confiscation of balances, creating a central authority that can whitelist and/or blacklist addresses, and requiring all users to register their wallets with a government agency. So far none of these have been implemented into the protocol but the pressure to do so is will only continue to increase, especially by venture-funded companies, especially in the USA."
If those things will happen to Bitcoin, I will leave it.
Im quite surprised that those kind of ideas are put on the table.. This is Anti-Bitcoin, and go against Bitcoin in its essence. The Bitcoin comunity can do without it, alternative will exist as demand exist. This is a shame that some are proposing such principle that are SO ANTI-BITCOIN..
If any instance, govt, powerfull entity ask for it to be apply to Bitcoin.. Why would we comply ? Afterall, Bitcoin are'nt just the best tool to avoid those kind of things (anti-lliberty, anti-privacy stuff) ?