F# is used in quite a number of firms, especially in the Chicago area I am aware of a handful of companies. Initially it was used just for prototyping/modeling type stuff, but more and more it is seeing the light of day in production. It will never see the adoption scale of Java/C# but that isn't the point or the goal.

It makes sense as well if you follow the history of most of these trading firms. Most of them started out just manipulating excel spreadsheets. Then brought in vb macros and such and then from there some diverge into java or go straight to some functional programming solution like ocaml with jane street, but others stay tied to msft with C#. Then F# comes along and allows intelligent developers at these msft firms to express themselves more clearly when dealing with the type of math problems they are solving.

If the firm is using C# I am almost certain that F# is in the mix there as well.

The F# user groups in London are filled with people working in finance who use F#. One or two people using it for science or startups (data analysis) as well. Looks to be a fair amount of interest in using it as a language for side projects for the people I spoke to there, while MS are not pushing it as much as C#, they do support its use in the banks.

In my experience, some funds may keep their technology choices close to their vest.

Jane street already use Ocaml, so they are probably lost to Microsoft. But that at least prove the presence of a market. Microsoft may want a share.

Credit Suisse has about 100 people in their algorithmic trading and risk management group using F#.

As NeXT proved, you can have good sales to high-end finance firms without being commercially successful.