Thanks so much for the feedback! I have a nephew with severe allergic reactions to tree nuts, shellfish, etc. so I guess I am probably being overly cautious about the wedding cakes. And since I do nut-based cakes for other occasions and use the same pans (even though I wash and sanitize them properly), it sounds like it would be in my best interest to provide what they'd like but make sure I have fully covered in the contract that I am not liable. I think I will also make tent cards for the bride and groom to set next to the cake and have them sign off that they received those as well. That way, if they decide not to put the cards on the table at least I have done everything I could to aid in letting the guests know the cake has a layer or layers with tree nuts (whether it be flavoring, in the cake or in the filling) and has possibilities of cross contamination.
After speaking with my nephew's mother, she agreed that they personally take responsibility to make sure there is no possibility of the ingredients, and if it is unknown but suspicious, they opt to not allow him to eat the questionable items.
Like I said, I would feel horrible if someone ended up having an allergic reaction and anything significant happened to them. But I guess that as long as I cover all my bases and try to help them keep the guests as informed as possible then that is the most I can do. Short of becoming a completely nut-free bakery which, as some of you have noted, is also kind of a sticky situation considering that lots of the ingredients have disclaimers about the processing and possible cross contamination, I see that is my best route.
Thank you all again!!