Not sure if you found the setting to turn off SMS forwarding, but to turn it off go to Settings on the top of the page, choose Phones tab, select your mobile VZ number and uncheck 'SMS Settings" box. If you don't do this you will get a GV app notification of the text and a VZ txt saying the same thing.
I still have my SMS plan from VZ because I have friends who don't use my GV number yet to text me. Eventually I may get rid of it on my plan. This will save me $10/month. More $$ to buy apps
That's how I remember seeing mine set up but when I look now it no longer looks like that. I see my VZ number and a check mark next to forward calls but there is nothing about SMS there anymore.
On the voice mail and SMS tab there is only a place to click to have your text sent to your email. I think something has changed on this page since I got my Droid.