Gee, that's almost exactly what I got:

  def flatten(nested_list):
      output_list = []
      for elem in nested_list:
          if type(elem) == list:
              output_list.extend(flatten(elem))
          else:
              output_list.append(elem)
      return output_list

I think that's easier to read than the following, and appears to run slightly faster as well?

  import collections

  def flatten(nested_list):
      if isinstance(nested_list, collections.Iterable):
          return [a for i in nested_list for a in flatten(i)]
      else:
          return [nested_list]