The tests shown here demonstrate the different qualities of radiosity with a light that has a fall-off distance that intersects geometry. The wall behind the camera is red RGB(200, 30, 30) and it was used to show noticable blotching.

Simply turned radiosity on (notice the intense red splotches on the wall and the ground):
[attachmentid=13783]

Added default final gathering:
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Set only Jittering to 100%:
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Set only Sampling to 500:
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The large red spllotches appear because some photons do not bounce off of the geometry correctly because the fall-off goes through it. The way to get rid of the blotches would be to decrease the light fall-off to the very first intersection of geometry, and then recalculating the intensity of the light.

Here's an example of what the radiosity in the room should have looked like:
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This image is done with only jittering; using only sampling will give you a clean render.