Curtis Price, you are correct, i.e., the OP's original syntax should work. I will have to update my comment to clarify that get/set applies to properties of the default extent and not the default extent itself. Basically, you can't modify the default extent properties in place, you need to modify an extent object and then pass it back to the default extent property to have the updates stick.
The following will not generate an error, but it doesn't update the default extent:
df = arcpy.mapping.ListDataFrames(mxd)[0]
df.extent.XMin, df.extent.YMin = -180.0, -90.0
df.extent.XMax, df.extent.YMax = 180.0, 90.0