Bitcoin does have an intrinsic value: the computing time it takes to mine a bitcoin.
Stop. Bitcoins have an intrinsic COST. The computing time that goes into producing a bitcoin is comparable to the paper and ink used to print a physical US dollar, or the wages and electricity cost of the "creation" of purely electronic dollars. (Which are really debts, rather than currency, but that's an entirely different boneheaded idea that the one you're postulating.)
Once a bitcoin is produced, it cannot be redeemed for an equal amount of computer time. In fact, using a bitcoin requires SOMEONE ELSE to pay for the verification chain that makes this electronic currency at all feasible.
(you're right that it's a bubble, and I'm not here to argue the system's inherent merits or flaws.)
Having said all that: the dollar, in contrast, really does have no practical intrinsic value. Ever since 1971, when Nixon threw the last vestiges of any standard away. (And defaulted on U.S. debt in the process, by the way. People who said the "fiscal cliff" would be the first time the U.S. ever defaulted on debt simply don't know their history.)
The US dollar is backed up by an almost non-intuitive fact of modern society. It's legal tender for payment of debts. As in, if you don't pay your employees or pay for that meal in a restaurant, or if you just wrong someone more generally, the courts will denote whatever judgement is finally ordered against you in US dollars, and if your wealth is denominated in some other currency, you'll be subject to whatever market exchange rate you can manage to produce sufficient dollars to pay the debt.
Or, in short, "people who say the US dollar isn't backed up by anything don't know what they're talking about."
On a different note, though, I'd be interested if you could point to a US debt that was denoted in a weight of precious metal and not redeemed for sufficient value to satisfy the bond-holder. Just because in 1971 the President of the United States stopped offering gold for dollars doesn't mean the US "defaulted" any more than Wal-Mart selling out of ammo means they "defaulted' on that gift card you bought. (The phrase "Redeemable in gold on demand at the United States Treasury, or in gold or lawful money at any Federal Reserve Bank" was gone from bank notes decades before. And still did not specify the amount of gold.)