Named arguments are a little-known feature of F#, that is actually very useful when calling heavily overloaded members. The DateTime constructor is one such example: it has twelve overloads, and I can never remember what (in C#) DateTime d = new DateTime(2008,11,12) means. Is it December 11th or 11 November (as we would say it in Dutch)? Fortunately, in F# there is a nice solution for this: you can always use the names of the formal arguments to specify a named argument. This not only clarifies what overload you're calling, you can also easily change the order of the arguments. For example:
let d = new DateTime(month=12, day=11, year=2008)
let d' = new DateTime(day=11, month=12, year=2008)
For F#, it's all the same.
You can also mix positional with named arguments, and if the object has setters, you can use these to. For details, read Section 8.4.5 in the F# (draft) language spec.