Before describing the problem, let me first point out that this is a distinct issue from this question.
The Problem
This screenshot was taken with a break set at tableView:didSelectRowAtIndexPath:, and as you can see in the simulator (far right of the image), there's a single-pixel blue line at the bottom of the selected cell. This is not the design asked for by the client, nor is it how this app used to behave: there should be no separator, even on selection.
How I Got Here
I'd initially designed this table view using custom UITableViewCell classes with corresponding nib (.xib) files and had no trouble with selections: the separator was hidden as desired. Predictably, scrolling was sluggish due to all the overhead from the view hierarchy, so I reworked the custom cells to use Loren Brichter's fast scrolling solution. Now scrolling is much faster, but I can't get rid of the separator for the life of me.
What I've tried
At the time of the screenshot above...
the table view has "Separator [None]" in IB.
the UIViewController that contains the table view has this line in viewDid Load: self.tableView.separatorStyle = UITableViewCellSeparatorStyleNone;
As you can see in the screenshot, I inserted some unused lines to prove that separatorStyle is set as desired. Other testing confirms that tableView and self.tableView are equivalent pointers at that same breakpoint.
I've also tried setting tableView.separatorColor to black and to clear, all with the same result: the cells look right until a selection is made.