At the moment, mono is the way to go - if you download mono and Xamarin Studio, you get a pretty good environment for doing F# on Mac. See the F# on Mac page on F# Foundation.
The fact that Microsoft is open-sourcing .NET and making it available on Mac too will be great in the longer term - it is already helping mono (because they replaced some of their libraries with the open-sourced versions from Microsoft) and it just gives additional guarantee that F# will work great on Mac and other platforms. But at the moment, this is quite early - and mono is already pretty high quality implementation of the runtime.
