Micro 4/3 The Samyang 7.5mm f/3.5 fisheye MFT lens looks like a winner.

LensTip has posted some sample images of the Samyang 7.5mm f/3.5 fisheye:

LensTip sample images of Samyang 7.5mm f/3.5

This lens is cheap, compact and relatively fast for a fisheye. I love the amount of curvature in the square format 180 degree fisheye - it is much better than some Sigma fisheyes I tried on my Nikon D90 recently and this lens will be a lot cheaper. The only downside that I see is the lack of electrical contacts with the camera body means no lens EXIF data.

Hmm, I think a fisheye will be in the shopping basked for my Olympus E-P3.