Statistics on MacBook Air failures?
I'm not aware of any available data on this. I'm sure Apple has detailed information on returns and repairs, but there's no way they would ever release it (and I'd imagine it's guarded fairly heavily even within the company). You might be able to get some numbers from independent Mac repair shops, but without any information on how many Airs are out there and how many are going elsewhere for repair, it wouldn't be all that valuable statistically.

The best potential source I can think of is some of the market research companies, like Gartner. I don't know if any of their surveys would be specific enough to get data for failures for individual models (or whether their sample sizes are large enough to make any inferences significant). If they do have such data, it would almost certainly be in their for-pay reports (the free ones typically only include the general sales figures, nothing with the specificity you're looking for).