> Well, the first thing to check for is Py_Py3kWarning. Then do the
> extra logic and execution speed.

In 3.0, it's always a PendingDeprecationWarning, so that won't work. 
The test needs to be:

if not recursing and warning_is_not_suppressed:
    warn()

The recursion test is expensive if using thread local storage; the
warning suppressed test looks expensive, too.  So there's no quick short
circuit test that I see.  Of course all of this is just hot air until
coded and benchmarked.  I'll cook up a patch, but it will probably not
be ready before the next alpha releases.