I used to have that problem when defishing. Whether using the lens correction tool in CS3 or the PTLens plug-in, I had to compromise between removing distortion and loss of image detail ... the more distortion you remove using these "rectilinear" methods the more detail gets "smeared."
I read a post in this forum from a member who found this great plug-in called "Fisheye-Hemi" from Image Trends:
Image Trends - Fisheye-Hemi Plug-In - Corrects Fisheye Lens Distortion and retains Image Content Details
At first I was skeptical, but after downloading the demo I realized this plug-in really does work as well as it looks like it does in the advertising. Here is a link to a fisheye image I took with an 8mm Sigma fisheye:
JPG Magazine: Photos: "Original Tasty Jam" by Jerry Jackson
Here is the same image after removing the distortion with Fisheye-Hemi:
JPG Magazine: Photos: "Tasty Jam" by Jerry Jackson