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.
As of iOS circa 4.0, there are specific instructions in the iOS docs that make this work super-fast:
http://developer.apple.com/library/ios/#documentation/UserExperience/Conceptual/TableView_iPhone/TableViewCells/TableViewCells.html#//apple_ref/doc/uid/TP40007451-CH7
Scroll down to where it talks about subclassing UITableViewCell.