Also, FWIW, if you like the ANSI version, it is easy to implement. Here is a version using a dead forked dialect of Squeak; it should be easy to dust it off should anyone want. This version goes further than discussed in this thread, and even makes floats and large integers be immutable. :) The code is in islands.zip on the following page; look inside the zip for immutLits1.5.cs and immutLits2.2.cs.
In 3.8 and 3.9 there are 5 subclasses of String (Symbol, Byte and Wide) and many more for ArrayedCollection. Your change would introduce many read-only classes and lead to much duplicated code (ok, traits would be a big help here).
Immutability should be an instance-level property and not special class. Immutability is completely ortogonal to inheritance und therefor should not abuse the inheritance mechanism. I know that a change like this would require some deep changes to the object representation in the VM. I just hope that someday somebody dares to make the step from 3.x to 4.0 where something like that is maybe possible ...
Cheers, Lukas