Do you honestly think the US government is going to allow a significant number of retailers to accept bitcoins?
Why not? The currency is public and traceable, and no one seems to have objections to Microsoft Points, FarmVille credits, Linden$, etc.
How, exactly, do you ensure compliance with tax laws when people are spending bitcoins? You don't. And the IRS isn't going to be understanding about that.
How, exactly, do you ensure compliance with tax laws when people are spending US paper dollars (cash)?
(rhetorical question...)