It looks like pip is grabbing the first .tar.gz package from PyPI. For nltk, this is the macosx binary. You will need to explicitly point to the correct package.
The easiest way to do this is to simply provide the full path to the package.
pip install http://pypi.python.org/packages/source/n/nltk/nltk-2.0.1rc1.tar.gz
Another solution is to download the package to a known directory and install it there. For example, let's say you upload a package to ~/Downloads . The command will be as follows:
pip install ~/Downloads/nltk-2.0.1rc1.tar.gz
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