As part of my testing of this lens I did some infrared shooting with this lens and my IR converted Canon 350D.
Well, as I naturally do, I try everything in infrared, as well as visible. So naturally when I got the new Sigma 10mm F2.8 EX DC FISHEYE HSM lens in for testing I naturally had to try it out with my infrared converted Canon 350D.
The 10mm works as well as the Sigma 4.5mm lens does. Unlike the 4.5, the 10mm covers the full APS-C sensor size.
Like the 4.5mm circular fisheye, the 10mm is sharpest in infrared in the f8-f11 f-stop range. It should not be used at f2.8 in the infrared, as the results are so blurry as to be unusable.
This is a great lens. It can be used in many more situations than the 4.5mm can and produces much more natural results.
If you are in the US you can buy the Sigma at B&H Photo.