Coming from a organization that is both services and a software company...
1) Is C# now widely used for Business Rules? No. VB.net is still preferred by most finance/functional/consultants because it is most like VBA in excel.
2) Is there any plan to support C# in member formulas? - I hope so
3) For those who moved their business rules from vb.net to C# - why you did it and what benefits you get out of it? I prefer to write in C# because the language is being evolved more than VB.net. I also prefer a more ANSI based language similar to C/C++/Java than VB's unique syntax and structure. Its about productivity. Use the language you can be most productive and creative with.
I use C# to write production code for software products. It's more difficult for me to think in objects using VB, so I build projects and products around C#
4) In OneStream - is there anything that one can do in vb.net but can't in C#? If so, what are they? - Agree with Jack, both languages support the platform equally.