Yes, it is a common behavior of the 18-200 and many heavy super zooms. As the lenses get used, they also tend to loosen up.
The new version of the 18-200, and most other super zooms have a lock to keep the lens in the collapsed position for travel, but that doesn't help when you are shooting.
You could certainly send it in for service to see what they can do, but I doubt if that is cost effective. Many people simply put a wide rubber-band around the lens barrel overlapping the zoom ring and lens body to control the creep. Some company is selling something like this, but basically it is a very expensive rubber band.