Recently there was a story of a ten year old Lab who got his tongue stuck in a toy called a Pimple Ball from Four Paws company. It's a very sad story, by the time the vet got the ball cut off his tongue, his tongue had been without oxygen for so long that it had to be amputated. The owner is understandably furious with the company and wants the toy re-called. She has a point, the ball has a single hole and it does create a vacuum, which really does represent a significant danger to a dog.
If you have one, I strongly suggest you get rid of it.
However, the more important point is that ANY ball-shaped dog toy with a single opening can do exactly the same thing - create a vacuum that will hold your dogs tongue in. So either get rid of the toy(s) or drill/cut a second hole to prevent the vacuum from occurring.
You can read more about it here: http://www.petsugar.com/1885759
and here http://www.fourpaws.com/products/pimple-ball-with-bell.htm
