There's really no precedent for any currency being "made illegal" nor is there any real push by any government to do so. Even the two senators (who are, TWO senators) who didn't like Silkroad didn't make any statements that they intended to create any sort of anti-bitcoin laws. The ability to criminalize possession, use or acceptance of a currency would be a massive expansion of government powers, which admittedly hasn't stopped the federal government in the past, would nonetheless be extremely unpopular with the current anti-big-government Teaparty lean of the populace. I don't think anything's happening to the legality of bitcoins themselves for many years.