I can confirm that Verizon does not charge for SMS through Google Voice. It is free to send and receive them. I checked my SMS count right before I started using Google Voice on the Droid I had for three weeks (just returned so I can get the Incredible tomorrow). Three weeks later, after sending and receiving many texts, the count was exactly the same.
Bottom line? If you are okay with using Google Voice as your SMS app, and can get everyone to send texts to your Google Voice number, you have free SMS and no need to pay for a text plan with Verizon. So you've got that going for you, which is nice.