NOTE: This doesn't appear to be working in iOS7 and above. For iOS7 try this answer.
For iOS6 and below, to remove the grouped background from a cell in a grouped table view cell:
This didn't work
cell.backgroundView = nil; // Did Not Work
This did
cell.backgroundView = [[[UIView alloc] initWithFrame:CGRectZero] autorelease];
If you have moved to ARC (I've heard this works, but haven't tested it)
cell.backgroundView = [UIView new];
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