Doctest output scrupulously follows the output of an interactive session
of python.

On my machine:

Python 2.4.4 (#71, Oct 18 2006, 08:34:43) [MSC v.1310 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy
>>> numpy.array2string(numpy.arange(2))
'[0 1]'
>>>

The the interactive session does not simply print objects, it calls
repr() on them, so that you can make the distinction between the number
0 and the string "0".

You should add quotes to your doctest string, or use print:

>>> import numpy
>>> print numpy.array2string(numpy.arange(2))
[0 1]