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I want to see this patch go in:  it affords some nice 
simplifications in many places, so should make the code 
easier to maintain and to change.

Some gripes:

+ Most of the new functions don't appear to be documented 
at all, except in that they're mentioned in passing by the 
docs for other new functions:  PyObject_GC_New, 
PyObject_GC_NewVar, PyObject_GC_Resize, PyObject_GC_Del.

+ Unlike the old Init/Fini, the new Track/Ignore 
don't "sound like" matching brackets.  TrackOn/TrackOff, 
TrackStart/TrackStop, Track/UnTrack, ... would be better 
for this reason.  I draw the line at PyObject_GC_New vs 
PyObject_GC_Old, though <wink>.

+ Overall, and despite the pain it causes, it's probably 
better to let GC continue to get triggered by "deallocation 
deficit", rather than moving the prod into the eval loop.  
Guido says you two already discussed that, so I won't 
belabor it here.  I'll note that one consequence of the 
current policy is that GC glitches got triggered during 
compilation, and mysterious as those were, that they *did* 
happen during compilation allowed to rule out huge pieces 
of the Python runtime code; it was significantly harder to 
pin the blame for glitches that didn't happen until runtime.