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Okay---this one I *can* reproduce. :)
I'm going to call it a gcc optimization bug. Specifically, it seems to be a bug involving gcc's builtin version of the copysign function.
When I build a current svn trunk checkout (r82245) with:
 CC='gcc-4.4 -m64' ./configure && make
I get the wrong result:
Python 2.7rc2+ (trunk:82245, Jun 26 2010, 05:35:07) 
[GCC 4.4.4] on sunos5
Type "help", "copyright", "credits" or "license" for more information.
>>> (-0.0).hex()
'0x0.0p+0'
But when building with either:
 CC='gcc-4.4 -m64 -fno-builtin-copysign' ./configure && make
or
 CC='gcc-4.4 -m64' ./configure --with-pydebug && make
I get the expected results. If I have time I'll investigate further and see if I can generate the bug from smaller code. At any rate, I don't think this is something that can sensibly be fixed in Python itself, so I think this issue should be closed, and a bug filed upstream if necessary.
I also can't see a good reason why this bug would be specific to OpenSolaris. Does anyone have gcc-4.4.4 available to test this on OS X, Linux or *BSD?