I presume this applies to builtin str methods like .lower(), right?  I think it is a good thing to do for Python 3.3.

We'd need to define what should happen in edge cases, e.g. when (against all odds) a string happens to contain a lone surrogate or some other code point or sequence of code points that the Unicode standard considers illegal.  I think it should not fail but just leave those code points alone.

Does this require us to import more data files from the Unicode standard?  By itself that doesn't scare me.

Would this also affect .islower() and friends?