From what I understand and from my limited experience with IB's(not to be confused with Interactive Brokers), the funding is held at the FCM, not the IB. I believe the IB has a sort of commission arrangement with the FCM and "piggy backs" off the FCM's reputation and financial solvency. For this reason, I am 80% confident that you will never have to place your money in the IB's account, but rather will have your funding held at the FCM for the Benefit of xyz corporation, client abc...
So I believe your main concern should be with the FCM...Fortunately, there has been alot of consolidation, so some firms such as Refco are behemoths with alot of solvency...