I don't think that the government could tax it, as it's not money :)
I'm not a lawyer or anything even close, but if your income is not money, but something else (e.g. potatoes), would that be taxed?
Bitcoins seem like capital items to me and I fully expect governments to treat them as such. If bitcoins become widely used enough people who haven't been careful will be facing investigation by the IRS or equivalent in their own country. It would not surprise me at all to hear of some large confiscations of computer equipment and bitcoin wallets by the IRS at some point in the future.
