Hi Tim.
I to am after a fish-eye lens and have decided upon the Tokina atx 107 10-17, a friend has this lens and I've borrowed it often and am delighted with the effect that it gives. It is, after all, just a spare "fun" lens, not a daily use one.
With my birthday next week I expect to find one nicely wrapped up on my pillow - God knows I've dropped enough hints.
Results of this lens can be seen here
http://www.tokinalens.com/products/t...07-10-17mm.jpg
In my lens arsenal I have a Sigma 10-20 which is perfect for wide angle shots on the 40D - I would never call the 10-20 lens a fish-eye, it is however super wide which suits me often.