To obtain a true fisheye effect, use the free Panorama Tools (
http://www.path.unimelb.edu.au/~dersch/) plugin to convert from rectilinear to fisheye projections.
A similar result can be achieved with my UnDistort plugin (
http://flofilters.free.fr) — the plugin is ment to correct barrel distortion of rectilinear lenses, but you can use it to do the inverse 🙂 The plugin is a shareware but fully usable for small images.
Florian.