OptionalAttribute parameter's default value?

MSDN's VS2010 Named and Optional Arguments (C# Programming Guide) tells us about optional parameters in C#, showing code like I'd expect:

public void ExampleMethod(int required, 
    string optionalstr = "default string", 
    int optionalint = 10)

Ok, but it also says:

You can also declare optional parameters by using the .NET OptionalAttribute class. OptionalAttribute parameters do not require a default value.

I read MSDN's OptionalAttribute page, and done searches online (which shows lots of people claiming OptionalAttribute parameters can't be consumed by C# -- I'm guessing these comments were made before C# 4?), but I can't find the answer to two questions:

If I use OptionalAttribute to define a C# parameter as optional:

1. what value will be used if I call that method and don't specify that parameter's value?
2. will that value be evaluated at compile time or runtime?