For the past couple of days, I've written a bit about my own responses to getting glutened. As a wrap-up, here are my suggestions for contacting the eatery where it happened:
(1) Maybe wait a day or two, until your body and emotions are a bit more stable. You'll be able to get your point across in a more constructive way.
(2) If the restaurant has an email address listed, send them a note along the lines of:
"Hi there. I was at your restaurant for dinner at 8pm on 6/2/11. I chose to eat there because you have a gluten-free menu. I made my food restrictions very clear to the server before ordering. Unfortunately, I became very ill after the meal. Since you offer gluten-free food, I thought you'd want to know about this event."
If you want to discuss it with them further, say so and leave your preferred contact info. If not, at least the incident has been registered. (Thus reducing the chances of them using the "But no one ever complains" response with a future GF diner.)
(3) If it's convenient, stop in to tell the owner/manager. I'd suggest going during down-time, i.e. before the dinner rush and not on a weekend.
(4) A phone call stating what happened works fine, too. At the very least, it will be a red flag for the management and staff that they need to be more careful in their GF processes.
I know that others have used social media, i.e. Twitter, Yelp, Facebook, etc. to publicly state where they've been glutened. I have not done this, but am curious to hear thoughts on that approach.
However you do it, I'd encourage you to contact them, period. No matter how you do it, they're not going to know that their food isn't safe unless they hear it from their customers.
Here's to safe and happy eating!