>Anyone has an example of going from System.Collections.Generic.IEnumerable, of type String, to a string?

Use String.Join() method as Sarosh already said. It doesn't need to be an array though and it works with IEnumerable. ie:

dim myString = String.Join(",", myStringList)

If it were a collection of some other type, you could still do that using ToString() method in between:

Dim myList = {1, 2, 3, 4, 5}
Dim Result = String.Join(",", myList.Select(Function(f) f.ToString()))