Here it is, the "great red spot" exposition...
Best viewed by looking at the "original" size, note the magenta spot at upper right. This appears to be heating of the 5D's sensor. It's the only spot I've found so far, but have not worked at ISOs over 800 or times over 6 minutes. This particular (RAW) exposure was 3 minutes @ ISO 800 using the Canon 100mm macro.
Exposure was converted in ACR 3.2 with both sharpening and color noise reduction off. Post processing in PS CS2 included only USM (250, 0.2, 3), curves, and resizing.

Here it is, the "great red spot" exposition...
Best viewed by looking at the "original" size, note the magenta spot at upper right. This appears to be heating of the 5D's sensor. It's the only spot I've found so far, but have not worked at ISOs over 800 or times over 6 minutes. This particular (RAW) exposure was 3 minutes @ ISO 800 using the Canon 100mm macro.
Exposure was converted in ACR 3.2 with both sharpening and color noise reduction off. Post processing in PS CS2 included only USM (250, 0.2, 3), curves, and resizing.