If you don't have such a package, you can try:

def json2xml(json_obj, line_padding=""):
    result_list = list()

    json_obj_type = type(json_obj)

    if json_obj_type is list:
        for sub_elem in json_obj:
            result_list.append(json2xml(sub_elem, line_padding))

        return "\n".join(result_list)

    if json_obj_type is dict:
        for tag_name in json_obj:
            sub_obj = json_obj[tag_name]
            result_list.append("%s<%s>" % (line_padding, tag_name))
            result_list.append(json2xml(sub_obj, "\t" + line_padding))
            result_list.append("%s</%s>" % (line_padding, tag_name))

        return "\n".join(result_list)

    return "%s%s" % (line_padding, json_obj)

For example:

s='{"main" : {"aaa" : "10", "bbb" : [1,2,3]}}'
j = json.loads(s)
print(json2xml(j))

Result:

<main>
        <aaa>
                10
        </aaa>
        <bbb>
                1
                2
                3
        </bbb>
</main>

Soviut's advice for lxml objectify is good. With a specially subclassed simplejson, you can turn an lxml objectify result into json.

import simplejson as json
import lxml

class objectJSONEncoder(json.JSONEncoder):
  """A specialized JSON encoder that can handle simple lxml objectify types
      >>> from lxml import objectify
      >>> obj = objectify.fromstring("<Book><price>1.50</price><author>W. Shakespeare</author></Book>")       
      >>> objectJSONEncoder().encode(obj)
      '{"price": 1.5, "author": "W. Shakespeare"}'       
 """


    def default(self,o):
        if isinstance(o, lxml.objectify.IntElement):
            return int(o)
        if isinstance(o, lxml.objectify.NumberElement) or isinstance(o, lxml.objectify.FloatElement):
            return float(o)
        if isinstance(o, lxml.objectify.ObjectifiedDataElement):
            return str(o)
        if hasattr(o, '__dict__'):
            #For objects with a __dict__, return the encoding of the __dict__
            return o.__dict__
        return json.JSONEncoder.default(self, o)

See the docstring for example of usage, essentially you pass the result of lxml objectify to the encode method of an instance of objectJSONEncoder

Note that Koen's point is very valid here, the solution above only works for simply nested xml and doesn't include the name of root elements. This could be fixed.

I've included this class in a gist here: http://gist.github.com/345559