I tried F# on the iPhone (via MonoTouch) tonight. The trivial hello-world code seemed to work just fine. I used standalone to build the (trivial) fsharp code:
type Foo() = do System.Console.WriteLine("This output brought to you by the note F#");
mono ../FSharp-22.214.171.124/bin/fsc.exe -a Foo.fs
And added a line to instantiate a Foo in FinishedLaunching().
Loaded the resulting dll into MonoDevelop, along with FSharp.Core.dll, and it just worked. Or at least it worked in the simulator, haven't tried the real device yet.