Immutability is a very powerful idea when record keeping and dealing with transactions. I know of at least one major bank whose ledgers are written Haskell.
I tried unsuccessfully to get a job with them about 4-5 years ago. They are a huge Haskell shop.
I simply didn't have enough background in Haskell. The interview was fun. The interviewer did his PhD in Haskell and compiler theory.
Around the same time, I applied to Facebook's Haskell group, and I got to interview with Simon Marlow, which was great. He also said that I didn't have enough Haskell experience and suggested that I do more Haskell consulting work, and re-apply in a year.
Anyway, I love coding in Haskell but my skills need improvement.