The fish-eye is less pronounced because you don't see the edges. Think of it this way - a fisheye lens designed for a film camera has less of an effect on a DSLR because of the crop factor - the most distorted areas are not captured by the sensor.
It's exactly the same with the fisheye zoom - at 10mm, you're getting the full 180' fisheye range (maybe fractionally less due to 1.6 crop rather than 1.5), but as you zoom out, you're essentially zooming into the image and the heavily rounded edges are lost, giving you only mild distortion at the edges.
For example... here's a shot (with unflattering lighting, unfortunately) from my 16mm Zenitar fisheye, which will be fairly similar to what you'd get with the 10-17mm when zoomed in. It'd take a close look to have any idea that this came off a fisheye lens. (This is the Saratoga Springs Resort at WDW.)
A Zenitar is a low-cost option to get some fisheye, but of course you don't have the option of getting the full effect on an APS sensor. On the upside, it's quite a nice lens and fairly fast - unfortunately, the 10-17mm is a little slow. But the ability to zoom really adds a lot of flexibility.
BTW, you might be interested to read Herb Keppler's comments on the lens. The original Pentax 17-28mm zoom was one of his all-time favorite lenses,
here is him talking about the joys of a fisheye zoom, and
here is him talking about the Pentax-Tokina alliance and specifically the 10-17mm lens, including a link to their review of the Pentax version.
I do really wish that it had a distance scale like the Zenitar does, but oh well... that's hardly a gripe limited to the 10-17mm!