I decided to take a crack at this today and here's what I came up with. First I took a short, 9 second clip from our recent film in .avi format and began experimenting with it in CLab Pro to produce the fisheye lens effect.
Here's a still shot from the original clip.
First I tried a displacement map composite as Mugsy42 did. I took two layers of the same clip and applied a feathered circle mask over the top layer along with a displacement map with the vertical and horizontal displacement set up to 146. Anything less than that didn't seem to produce enough distortion.
As you can see, I found the results unsatisfactory. It sort of produced a fisheye lens effect, but the distortion was uneven going out from the undistorted center, no matter how much I played with the feathering.
Next I decided to scrap that approach and tried a radial blur. For this run I applied the radial blur to the bottom layer and then put an *inverted* circle mask with the same amount of feathering on the top layer. The results were far more satisfactory to me. I had to keep the amount of radial blur down very low, a setting of only 3, as anything above that took the image from fisheye distortion all the way out to "bad LSD trip lala land".
Here's a picture of the circle mask I used to get an idea of how much feathering was applied. (NOTE: the circle looks distorted into more of a vertical oval, but that's because the clip had been cut in WMM and it doesn't recognize widescreen until your final export, when it looks fine. It was actually a default, even circle.)Here's a short movie file I made as sort of a tutorial
showing the original footage and both of those effects, with title screens explaining the settings I used. That way you can see the effect on a live action clip rather than a still photo. It's one minute long and around 8 MB in size for easy download in .wmv format.
Hope that's of some help.