tedster - 6:01 am on Aug 12, 2011 (gmt 0)
If it happens to you (and it is becoming less common, in my view) keep your wits. If you document what is happening and submit a balanced Reconsideration Request that disavows any involvement in the activity, you can reverse the negative effects. I know of several successful cases.
The best defense is to build a solid "brand" yourself, including the kind of solid backlink profile that is free of any manipulative garbage.