Full disclosure: I have never liked the idea of the spot-on products. It always seemed to me that they could rub off.
Now reports from the Environmental Protection Agency seem to confirm my feelings.
It's certainly no surprise that dog products used on cats, or large dog products used on small dogs, would cause problems. These products are poisons, after all, or they wouldn't work on the fleas. The alarming part of the message, to me, is the new advice to keep dogs and cats separated after using the products. There would be no reason to do this except that the dog product could transfer to the cat, either by contact or by licking.
If this transfer can be made, then it is also conceivable that products could transfer to bed linens, clothing, or hands petting the dog. So are you now not supposed to touch your dog after treating them with a spot-on product? For how long? And if the product is meant to flow over the dog's entire body after being applied to only one or two spots (another fine point that has bothered me about these products), then does it also flow over human skin once it comes in contact?
I have written endlessly about flea control, and have always had a hard time recommending these products to anyone but a person with a dog with flea allergies. I had one of those myself, and it was hard to get satisfactory control (and I lived in California at the time, where the flea infestations are much worse than in the Pacific Northwest), and still I didn't use the spot-on products on my own dog.
I use Program, which is not insecticide, but a sort of birth control for fleas. Its ingredient has no effect on mammals. True, it doesn't kill the fleas, and yes, we do have a few bites from time to time, but it is effective in not letting an infestation take hold. I am happy with its results, and very happy not to be putting a poison on my dog every month.