I am able to design custom UITableViewCells and load them just fine using the technique described in the thread found at http://forums.macrumors.com/showthread.php?t=545061.
Actually, since you are building the cell in Interface Builder, just set the reuse identifier there:
Or if you are running Xcode 4, check the Attributes inspector tab:
(Edit: After your XIB is generated by XCode, it contains an empty UIView, but we need a UITableViewCell; so you have to manually remove the UIView and insert a Table View Cell. Of course, IB will not show any UITableViewCell parameters for a UIView.)