If bitcoins are not a currency, then they are a good. if they are a good, then all kinds of transactional taxes apply. in the US you may have to charge sales tax, in europe a "miner" is basically "creating" bitcoins which, if they are a good, require VAT to be collected on a sale. SIGNIFICANT disadvantage - suddenly you loose a significant part, theoretically, of your income, unless you sell them out of europe
I operate a mining business: I produce a commodity for which I am paid in U.S. Dollars. This is no different from being a farmer or mining actual gold. If I hold some of my BTC it's roughly the same as keeping some of my crops/gold for personal use or as savings. All of the above scenarios have specific tax statutes which apply to them and which should apply to bitcoin until such time as it becomes regulated otherwise. Since tax law currently views me in the same light as a farmer or (gold) miner, my taxes will be prepared and paid in precisely the same manner.
For investors, it's much the same as trading stocks, bonds, commodities or FOREX. Gains are not considered "realized" until BTC are converted back into USD and captial gains taxes are to be paid on any realized gains as normal.
I don't understand why everyone thinks of bitcoin any differently right now. It's essentially the same as a commodity market: people produce it, people use it as an investment vehicle and people use it for barter - there are specific tax laws for each and every one of these scenarios (at least in the U.S.) with regard to other commodities and at present they're quite likely to apply directly to bitcoin.