I'm not sure that's the case... you might not be changing the value of the reference, you will essentially be assigning the local reference (the parameter reference) to null, not the original member object.

Think about this:
```
class A { var txt:String; }
class B { function changeTxt(txt:String) { txt = "World"; } }
function main()
{
 var a = new A();
 var b = new B();
 a.txt = "Hello";
 b.changeTxt(a.txt);
 trace(a.txt);
}
```
I'm actually not quite sure what will happen in this case...