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Jools,
I have not heard that the 5D Mk2 was any more affected than other Canon camera bodies.
Generally, grease that gets transferred over the sensor comes from moving parts that were lubricated in the factory. I understand that this used to be a problem, but is less so now as the lubricants used latterly have a greater propensity to adhere to these moving parts rather than getting splattered around the internals of the camera body. Perhaps your version was lubricated by an over zealous operator in the factory, which could be put down to bad luck.
It is possible that you may have inadvertantly come into contact with a lubricated area of the camera body with the tip of the Arctic Butterfly and then transferred grease from the brush to the sensor's glass shield. Either way you've been unlucky, but your solution seems to be an effective way of restoring the sensor back to tip top condition.
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