Taking a quick peek at my own code, the only place I see that strings are raised is in code that works with ZServer (e.g. "raise 'redirect'"). I was not speedy to get rid of string exceptions in my own code, but except for external compatibility, I think they are all gone now. Does Zope (clearly the 800-pound gorilla in most backward compatibility discussions) still use them heavily?
Zope 2 uses them heavily. But I doubt it will ever need Python 2.4; more likely, it will settle on Python 2.1.3 (if/when that is released).
In Zope 3, string exceptions are officially out of grace. (I don't know if there are any left, but if so, they will be removed soon enough.)
--Guido van Rossum (home page: http://www.python.org/%7Eguido/)