This is just my educated guess but I believe I'm right for this explanation.
Charles even asked during one interview on C9 called Jimmy Schementi: Inside IronRuby so here's my answer:
The CLR is a staticly typed runtime, which is also known as being "strongly typed". Ruby and Python are Dynamic languages which are primarily typeless. SO for the dev's making their .NET implementation wanted another way to say or prefix the Languages names with a "Strong" metaphor. What better to depict strength other than Iron??
So I believe that naming a dynamic language ported to a strongly typed .NET CLR is called Iron* specifically for that reason, thus the name IronPython and IronRuby were born.
Any thoughts ??