That's funny--as a python guy I was squirming for a set comprehension immediately:

  >>> a = {'a':1,'b':2}
  >>> b = {'b':3,'c':4}
  >>> def keylist(*maps):
  ...     return ', '.join(sorted({str(k) for m in maps for k in m.keys()})) or '<none>'
  ...
  >>> keylist(a, b)
  'a, b, c'
  >>> keylist()
  '<none>'

I'm sure people will have differing opinions on the readability of a nested set comprehension, but from the bullets at the top of the article to one-liner implementation in ~30 seconds. (For an arbitrary number of maps, no less.)

(edit: added '<none>' on empty, fixed formatting, whoops now I'm over 30 seconds :))