Answer from the manufacturer (posted to an email list)

We're glad you came to us for the facts about Swiffer WetJet. All our products have been evaluated by internal and external
veterinarians and scientists, and Swiffer WetJet and Swiffer Wet cloths are safe to use around pets. Let us assure you, this rumor is completely false. Our Wet cloths and WetJet liquid solution cleaners
do not contain antifreeze or any ingredient similar to it.

We have pets too and their health is very important to us. We hope you'll help us put an end to this rumor by letting others know the
truth.