On Fri, 1 Feb 2002, Martin McClure wrote:

> I'd like to have a bit in the object header for the purpose of
> marking an object immutable.
>
> Usefulness:
> There are two basic classes of uses for this capability.
> 1) Preventing the modification of an object
> 2) Detecting the modification of an object

Hear, hear.  I was just about to propose the same thing myself.