It has now been confirmed by ngzhang that BFL is using Altera Stratix III EP3SL150 FPGAs in their singles. Since Altera has made product reliability data available, we now at least have the appropriate info available for the chips themselves.
The EP3SL150 testing gives it a FIT rating of 15.1 which corresponds to roughly one failure per 66,226,166 hours or ~7,555 years. I don't think we need to worry about the FPGA being the point of failure.
That said, we have precisely zero data about the other parts or failure rates thereof nor do we know how often BFL messes up on their soldering, power supplies or any of the other links in the chain that could contribute to the overall failure rate of the BFL single. I did, however, email BFL and recently received this email from Nasser G at BFL:
No Failure data is available. So far, none of our units have failed,
and all units go under strict quality control measures before they are
shipped. We've had like 4 or 5 PSU failures among all the units that
Of course this is from BFL's mouth so whether it's entirely true or just marketing I can't say with certainty. Lacking data on the overall number of units sold or how long those units have been running (though we could estimate that as 24/7 since they've arrived if we had the other data) it's very difficult to estimate failure data based on BFL's comments, even if we assume them to be completely accurate.