This is a known problem, that's unlikely to change unless there's a 
serious overhaul of Python's floating point.

The warning at the end of the math module documentation should probably 
be taken to apply equally to all the built-in floating-point operations:

"""The math module consists mostly of thin wrappers around the platform 
C math library functions. Behavior in exceptional cases is loosely 
specified by the C standards, and Python inherits much of its math-
function error-reporting behavior from the platform C implementation. As 
a result, the specific exceptions raised in error cases (and even 
whether some arguments are considered to be exceptional at all) are not 
defined in any useful cross-platform or cross-release way. For example, 
whether math.log(0) returns -Inf or raises ValueError or OverflowError 
isn't defined, and in cases where math.log(0) raises OverflowError, 
math.log(0L) may raise ValueError instead."""

Or, as Tim Peters is fond of saying:

"""All Python behavior in the presence of infinities, NaNs, and signed 
zeroes is a platform-dependent accident, mostly inherited from that all 
C89 behavior in the presence of infinities, NaNs, and signed zeroes is a 
platform-dependent crapshoot."""