Hi -

I've never made potato soup, and am not sure what it's supposed to be like.
But as a rule I've found that you can substitute chicken stock, vegetable
stock, or fish stock (as appropriate) for milk in most soups.  You could try
it out.  It doesn't come out creamy (because it doesn't have milk protein)
but it does come out tasty.  (Casein is the protein that makes milk taste
smooth, and that makes cheese melt so nicely.  I don't know of a good way to
manage those textures without actual milk.)  BUt the recipes do come out very
tasty and satisfying.  For a somewhat creamier taste, try adding a bit more
of whatever fat you're using.  Not too much, though!

I still use the standard Toll House recipe for chocolate chip cookies.  I
use Crisco instead of butter (since Fleischmann's unsalted margerine has
disappeared from my neighborhood) and kosher-parve chocolate chips.  (Trader
Joe carries them; and I've had success with several store brands of chips, too.(

I hope these suggestions help,
--Beth Kevles
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  http://web.mit.edu/kevles/www/nomilk.html -- a page for the milk-allergic
  Disclaimer:  Nothing in this message should be construed as medical
  advice.  Please consult with your own medical practicioner.