I've heard that the following celebs avoid wheat. So they may or may not have Celiac Disease (some could just avoid wheat because they think it's healthier... or perhaps for religious reasons), but... they are supposedly on similar diets! Many of these I found on a message board at Celiac.com.
Billy Bob Thornton
Victoria Beckham (aka Posh Spice)
Regis Philbin (from Regis and Kelly)
Elisabeth Hasselbeck (The View)
Sarah Vowell (Violet in The Incredibles)
Terence Stamp (Jor-El in Smallville)
Keith Olbermann (MSNBC)
Dannii Minogue (Kylie Minogue's sister)
Scarlett Johansson said in an interview that she has a wheat allergy, referring to working in a wheat field. Not sure if she is on a wheat-free diet.
Gwyneth Paltrow has been on a GF diet in the past (not sure if she still is).
I thought that was pretty neat. I wish more would speak out about it though! I'm sure there are many more out there that we don't know about.