This is the logic that I've been operating on. Sure, you guarantee that you get the SIII and keep unlimited data by pre-ordering now, but Verizon will definitely announce a date before the change. Its not like we'll all wake up one day and its gone. The only risk is if they run out of preorders, then give a 2 week notice that goes into effect before the phone releases, or more preorders are available. Given the high profile of this device, I don't see them doing that.
I'm waiting to hear a date, and hoping that the Razr HD comes out before the switch as well. It's been "coming soon" much longer than the VZW version of the SIII, so it may drop before then as well. I have 2 upgrades available, and I'd prefer to burn one on a new Razr HD first, then get the SIII a few days later. As much as I prefer Samsung to Moto, from personal experience I can say I'd feel much better with having the extended warranty on a Samsung than a Moto, if I have to pick just one.