Just using Bitcoin for transactions doesn't eliminate legal obligations. I imagine that if they ever got popular, tumbling services would be the first things that would be regulated (or shutdown by the gov't).
The level of regulation it would take to actually trace lots of tiny transactions between pseudonymous parties is next to impossible. How is me paying a friend back for half the pizza we shared to a new address of his going to be handled?
It's pretty straightforward for the Government to come up with a scheme to defeat tumbling services. Just make it mandatory to only accept payments from whitelisted addresses. Banks and your average business are not going to go all cypherpunk and circumvent such laws.
Under my scenario, if you used zerocoin, and anybody else using the service wasn't whitelisted, you would be blacklisted, since there would be a chain of coins being sent from non whitelisted addresses to yours. So it would just be up to you not to use it.
The key is at what stage the Government becomes involved. If they implemented this sort of proposal right now, they would just say "if you're using zerocoin, you'll likely be blacklisted- tough luck". If everyone starts using zerocoin, including all legitimate businesses, and then they try to regulate it- then people would have more power.
Well, yes. Of course, it's not like tax authorities haven't had to deal with money laundering before.