This is what I did with WcfStorm.Server and WcfStorm.Rest (http://www.wcfstorm.com). The UI layer was written in C#; all other layers in F#. Switching back and forth though between an OO and an FP language (at least from my experience) is not that easy. There were a few instances where I ended up writing a class with mutable properties in F# simply because I didnt want to break my chain of thought so that I can get a feature out quickly
