SIPC doesn't insure all types of investments, I'm not sure if BTC would qualify. And if it doesn't qualify cash intended to purchase BTC probably wouldn't be insured either.
It was decided in the Shavers case that Bitcoin is a "security" or "investment contract" for regulatory purposes. The SIPC insures securities, including investment contracts, and cash you have with a US regulated broker, up to their limits of $500K securities/$250K cash. They don't do commodities. The SIPC successfully unwound Lehman Brothers, Madoff, and MF Global, and everybody got their money back up to the SIPC limits. Dealing with a failed Bitcoin exchange would be a small job.
The SIPC is good at finding and getting back assets that somehow wandered off. See "http://www.madofftrustee.com/". Some insiders who thought they'd gotten away with big gains from Madoff funds had them "clawed back" by the litigation trustee. Billions of dollars were clawed back.
Madoff himself is Federal Prisoner #61727-054, in a cell at Butner Federal Correctional Institution, scheduled for release in 2139. That's what should be happening to heads of Bitcoin exchanges where the assets just "disappeared".
