-so if the inspecting rabbi gave the product a D, he must have had a reason to feel there was a possibility of cross contamination.
This is a judgement call, to be made by the allergic individual (or their parents) based on their assessment of the potential risk.
We have a hair-trigger allergic child - in the allergic 2% - who is anaphylactically allergic to all nuts, eggs, and milk-derived foods. He has eaten several items labeled kosher-dairy without any dairy ingredients, and without any serious reactions. Occasionally he gets a little itchy, but nothing severe.
We always try new items in this category carefully. A little bite and then wait and watch for a few hours (although type 1 immediate hypersensitivity reactions usually occur in the first 1/2 hour).
It took us a while to get the courage to try such "oral challenges," but it turned out to be worth it.
On the comment about the rabbi inspecting and deciding on cross-contamination: this is probably not a completely accurate characterization of what happens. The rules and the procedures are rigorous, but a D is not an absolute declaration that the food contains dairy products or that it's been contaminated with dairy.
It certainly puts that food in a "suspect" category for us parents, but what it means to people who keep kosher is that it *might* contain dairy products or it *might* be made on dairy equipment, so to not break the laws of kashrut (about keeping milk and meat separate) it'd be best to only consume that food with dairy or pareve products, not meat products. A bit different from not eating it at all due to risk of severe reaction.
That's why the prior posted comment about these designations being for religious purposes, not allergy purposes. That's the difficulty with these things. We use these designations because they're the only uniform and somewhat reliable designations. But they fall short of proper allergen label standards, because they're for a different purpose.
As I've said before, there might also be economic considerations that lead a food producer to opt for a OU-D designations. They may want the flexibility of adding dairy ingredients later, or the flexibility of using the equipment for dairy.
As I said, though - it's up to your assessment as to what's safe and/or worth the risk. I don't know how old your child is, but we preferred waiting for our son to be old enough to describe how he felt, rather than just watching for symptoms.
I'm hoping we'll see steady expansion of allergen labeling, and even some uniform standards.
Good luck with all of this.
Mark