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> The pickle output has the sign-bit set. Ignoring the sign-bit, it is 
> unpickled correctly.
Okay, thanks for the clarification. I just wanted to be clear whether there's a real problem with pickle that should be fixed in 2.7 or not.
Again, I don't see this as a bug: pickle is transferring the sign bit correctly, so the only issue again is that float('nan') happens to produce a nan whose sign bit is set (depending on platform, compiler options, etc.). I don't see it as a problem that one can end up with some 'positive' nans and some 'negative' nans on a single system. As far as I know, none of this violates any documentation or standards (IEEE 754 explicitly places no interpretation on the sign bit of a nan, and makes no guarantees or recommendations about the sign bit of the result of converting the string 'nan'). So at worst, this is a minor violation of user expectations. Though I do agree that having float('-nan') having its sign bit unset is especially surprising.
So while I don't see this as a bug that should be fixed for 2.7, I'm happy to 'fix' this for Python 3.3, partly for the portability improvement, and partly to avoid having the string-to-float conversions do INF*0 calculations at run time; that bit's always made me feel uncomfortable.
Thanks for the updated patch; I'll take a look shortly.