Oblong Spiral Puzzle

Brian Jones posted a solution to the Oblong Spiral Puzzle recently.
I don't normally mess with puzzles, but after looking at his solution
I thought I would take a stab at it and see if I could come up with a
solution in Python that was a little less complicated.

The complete definition of the puzzle is on the Code Golf site.
The challenge is to fill in an n x m array with a sequence of
numbers starting at the upper left, and working around the outside,
spiraling inwards. For example:

$ python spiral.py 3 3
  1   2   3
  8   9   4
  7   6   5

$ python spiral.py 4 5
  1   2   3   4
 14  15  16   5
 13  20  17   6
 12  19  18   7
 11  10   9   8

Spend a few minutes thinking about the problem, then see if your solution looks anything like mine.

Updated to fix link to solution.