Brokerages are supposed to keep your money separate from theirs. So, even if they fail as a company, your money and investments are still there, and can be transferred to another brokerage. It doesn't matter if it's an IRA or taxable account.
Of course, as is the case with MF Global, if illegally take their client's money (i.e., steal), it may be a different story. In such cases, SIPC covers up to $500K, of which $250K can be cash, as JoeTaxpayer said.
You may be interested in the following news item from the SEC. It's about some proposed changes, but to frame the proposal they lay out the way it is now:
The most relevant quote:
The Customer Protection Rule (Rule 15c3-3).
This SEC rule requires a
broker-dealer to segregate customer securities and cash from the
firm’s proprietary business activities. If the broker-dealer fails,
these customer assets should be readily available to be returned to