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Arguably none because the JVM lacks tail calls and they are required to make almost all functional code robust with respect to stack consumption.
The nearest thing to functional language implementations on the JVM are Clojure, Scala and the OCaml-Java project. Although there are workarounds for the lack of tail calls (e.g. trampolining), none of these language implementations do this because the workarounds introduce even more serious problems, e.g. crippling performance and completely obfuscating debugging.
Sun have been talking about tail calls for years and, more recently, have indicated that they intend to implement them imminently. As soon as that is done, I am sure we will see a lot more language diversity on the JVM and, in particular, some production-quality functional language implementations. Until then, I regard all of these languages as toys.
Cheers, Jon Harrop.
Actually, I might be wrong, but I don't expect F# to be as mainstream as the other .NET languages; useful in a few circles (academic, compilers, a few other scenarios) - however, don't forget that C# offers FP usage - and it gets better each time: C# 1.2 has delegates; C# 2.0 has anonymous methods and captures/closures; C# 3.0 has lambdas for simplicity, and Expression for abstraction. Anonymous types (C# 3.0) share some similarity with tuples (in terms of convenience), but obviously are very different beasts, so definitely not a like-for-like comparison.
Maybe not quite as optimised as F#, but for most day-to-day FP use-cases, more than sufficient.
It is also quite clear that better support for immutability (especially for threading) is very much on the minds of the C# language team for future consideration.
My money is on C# getting better at FP, and being the .NET FP offering for most day-to-day purposes. Of course, there will be some F# usage - but (purely subjective) I simply don't see there being a huge migration.