I'm no matz or Guido (no surprise there), but I would point out that 
Apple's Cocoa has an immutable string class and a mutable string class.

As Robert Feldt pointed out, you can also implement a 'freeze' method, 
and if you really don't trust yourself and others, you can implement 
the Seattle Ruby Brigade's 'deepfreeze' method (part of their Ruby 
audit, which can be found here:

http://sourceforge.net/project/showfiles.php?group_id=50485 ).

Also, while I'm speaking out of ignorance and mere supposition, 
wouldn't immutable strings cause difficulties in languages with more 
than one writing system? In particular, Korean uses hangul (a phonetic 
alphabet) and hanja (Chinese characters) and Japanese uses kanji 
(Chinese characters), hiragana (syllabic symbols), katakana (symbols 
for foreign words) and furigana (keys to pronounce kanji). Hopefully, 
some with knowledge in this area, will correct or confirm my 
supposition.

Regards,

Mark