There is a JSON API for PyPI which allows getting data for packages:
http://pypi.python.org/pypi//json
http://pypi.python.org/pypi/
I know that you asked for a way to do this from the JSON API, but you can use the XML-RPC api to get this info very easily, without having to parse HTML.
try:
import xmlrpclib
except ImportError:
import xmlrpc.client as xmlrpclib
client = xmlrpclib.ServerProxy('https://pypi.python.org/pypi')
# get a list of package names
packages = client.list_packages()