Yes, this is entirely normal. It's especially common if you have a filter permanently on the lens. Nikon won't do anything about it, precisely because it is normal.
The best way to address the problem is simply to hold the lens by the zoom ring - then it won't zoom on its own. This doesn't help with a tripod, but because the main value of an 18-200VR is that it's convenient and handy, I'd be surprised if many people really have the tripod problem.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!