OK, some confused folks here:
1) Spending bitcoins on goods/services still requires you to pay tax just the same as if you cashed them. If you buy a computer with bitcoins that appreciated in value from $5 to $200 you have to pay taxes on that increase in value as if you had sold them for USD first and then bought the computer with USD. However, note that it is harder for the IRS to determine what you have purchased with bitcoins directly, especially if you order via Tor and have said goods shipped to a friend. Not that you should do that since that's illegal.
Note that I am not condoning tax evasion, even though some crazy people think it's our duty as patriotic Americans to evade the taxes imposed upon us by our corrupt, terrorist government who uses the money to kill innocent people with bombs all over the world and give ridiculous subsidies to oil, defense, nuclear fusion plants, Monsanto, and other nice people. Not condoning tax evasion at all. 2) Sorry, you can't just ACH money from your exchange account into your bank account and then not report it. Any large transfers could be reported by your bank to the IRS, and transfers above a certain amount are required to be flagged and reported. Also, the IRS could decide to audit you, and could request bank statements from your bank, and could ask why you didn't report the $700/week transfer coming from MyBitCoinExchange.net.
Fortunately, there is hope. You could OTC everything. That's harder, especially for those who can only afford to invest small amounts at a time. The risk there is unknowingly selling BTC to an IRS agent - and I wouldn't put it past them.
A slightly less secure option for buying - is to sign up for an exchange account and only ever access it from within the Tor network, and you could hop accounts frequently. Funding is the only issue. You'd have to appear in person at a bank to make cash deposits to said exchange, and if the IRS really wanted to get people, they could subpoena exchanges for records, subpoena BitInstant or LocalTill for bank cash deposit records, subpoena the banks for security camera archives, and get your picture from that. They could go that route in the future for very large evaders.
Of course, after buying at the exchange, you'd then have to sell OTC if you wanted cash, which brings the same risk mentioned before - undercover agents. You can partially avoid this risk by only selling to friends, which requires you have a significant network of trusted friends or family with enough capital to buy BTC from you when you wish to sell.
So, it would be a bit of work to effectively evade the IRS in regards to bitcoin profits, but as they tell us in state-sponsored kindergarten, "you can do anything if you put your mind to it."