>>> math.log1p(0.0)
0.0
>>> math.log1p(-0.0)
0.0

Ah, that would do it, then.  It looks as though the system's log1p function is buggy, in the sense that it doesn't follow C99 Annex F (F.9.3.9).  It also doesn't agree with what 'man log1p' says on my OS X 10.6 Macbook:  under 'SPECIAL VALUES', it says:  'log1p(+-0) returns +-0.'

I'm puzzled about one thing, though:  there's a test for this problem in the configure script, and LOG1P_DROPS_ZERO_SIGN should be defined in this case---in that case, the two testAtanSign and testAtanhSign tests are skipped.  So it looks as though LOG1P_DROPS_ZERO_SIGN isn't being defined on this machine;  I'm not sure why---there may be a compiler optimization kicking in in the configure test.  (Is this clang or gcc, BTW?)

So definitely worth a bug report, I'd say, though perhaps it's too much to hope for a fix within the lifetime of 10.8.