I am skeptical that those "only audible to dogs" whistles do anything at all because my dogs don't respond to them, but then again they pretty much ignore the "totally audible to everyone" variety as well. Now there are Soniks squeak toys which utilize the same frequency as those whistles- so your dogs can squeak away and you don't have to hear a thing.

Developed and tested in the Department of Speech and Hearing sciences at a major research university. The Sonik’s ultrasonic squeaker produces a sound with a frequency range between 25,000 Hz and 35,000 Hz - above the audible range of human hearing, but well within the hearing range of dogs.

The toys are a family of aliens from NoNoyz, the "quietest planet in the universe". Unless you're a dog, I guess. I think I will be able to tell if these toys work, actually, because my dog Shasta is obsessed with squeaky toys, and when I give her one it doesn't last long because she can't rest until after an exhausting bout of squeaking, she then mangles and de-squeaks it. A non-squeaking toy leaves her cold.

Soniks are priced from $4.99 to $9.99 and are available at most of the big pet stores.