Depends on the rhetoric swirling around in the bitcoin community at the time. It's possible that we could sell governments on the idea if we undertook to do so. Though bitcoins are a cash analogue, they are ultimately more trackable due to the existence of the blockchain. If a government were to get ahold of a mapping of addresses to identities, calculating tax owed by an individual from bitcoin activity would be as simple as mining the blockchain for data. If a country moved to using bitcoins exclusively as legal tender, they could feasibly replace most of their tax departments with a computer program.