Are you doing anything special in your -tableView:heightForRowAtIndexPath: method?
It looks to me like the height of the row is being set incorrectly, so the contents of the cell are expanding outside of its bounds.
I've had a similar problem. For me, the single line was caused by a superfluous view that was created but never sized or placed correctly and so was 1 pixel high, floating over everything else. You can also cause this by confusing a UINavigationController about its set of subviews (by adding views directly to its layout container).
Look through your UI (xib files and programmatically created views) for a view that shouldn't be there or is otherwise not being used. It might be helpful to write some code to dump a UI Hierarchy, so you can see what views are where.
The problem disappears when you set the cell height for the table view to 1 pixel in IB. It seems that before you populate the table, an empty table is drawn with the outlines of the cell height set in IB. Don't set the cell height to 0. IB doesn't like that. :-)