This is a Canon 1D(s) issue, but I know some M8 users have mentioned seeing evidence of lubricant spatter on their sensors, so it seemed appropriate to note that it DOES happen.
Support - Digital SLR - EOS-1Ds Mark III - Service Notices - Canon USA Consumer Products
Kudos to Canon for being on top of this issue, even if they have been knocked for being slow to respond to some things (1DIII AF issues, e.g.)
Kudos to Canon for the detailed and extensive explanation of what is going on - including the Mark-Norton-like "surgery" picture of the offending mirror hinges.
As Leica moves (we hope) towards a more professional support and service system to back up the S2, I hope in their visits to this forum they take note of this post. This is how to address the inevitable occasional problems with products!
----------------
P.S. I hope this will not be an issue with my forthcoming 5DII, but that if it is, I hope the 5D will get the same service and response as the 1D models.