Hi --
 
I have also heard that bedwetting may be allergic in origin.  However, most of the time it does not appear to be.  It's freqently just a matter of body maturation.
 
We tried an elimination diet for the child in my own family who was a bedwetter, with no result.  But recently the wetting went away, with no change in the diet whatsoever!  We'd expected this change, though, since late-wetting (ranging from stopping at age 8 to age 18) runs in the family.
 
If you google for "enuresis", which is the medical term, you can find more information.  In the meantime, buy a good night diaper, and take one week every summer to test going without it.  (The habit if wetting may persist in an older child, even if the need eventually passes.  Going diaperless at night for a few days lets the child figure out if s/he can safely sleep AND wake up to use the bathroom when needed.)
 
I hope this helps,
--Beth Kevles
  Eating without Casein website
  web.mit.edu/kevles/www/nomilk.html