Here's a list of toxic plants and it's not on it.
I've had verbena in many of my yards, and we always had dogs. We never had any problems with it.
Apparently Lemon Verbena can be toxic according to this article.
Toxicity to Dogs
While some species of the verbena family, such as lantana, are considered toxic to dogs, lemon verbena is generally safe unless your dog consumes large amounts. Known interactions can include kidney irritation, so you may want to reconsider planting lemon verbena if your dog is an avid chewer with kidney problems.
If your dog has consumed lemon verbena or any other of your garden plants and exhibits diarrhea, vomiting or unusually heavy panting, contact your veterinarian. The ASPCA provides an online database of toxic and nontoxic plants and offers a toll-free number to ASPCA Animal Poison Control. A consultation fee may apply.