Note: I purchased some M6 x 3/4" screws with clip nuts and nylon washers from Rackmount Solutions (P/N HWM6-50-Clip). I would have preferred to find the combination hex and Phillips screw head that IBM uses by default, but for the price I'll sacrifice having the hex head and go with the Phillips only. Both the screws and clip nuts are functionally interchangeable with the hardware that originally came with the IBM NetBay 42U rack.
According to the IBM Rack Installation Instructions it appears that they use an M6 screw; however, I'm not certain what length.
Note: I purchased some M6 x 3/4" screws with clip nuts and nylon washers from Rackmount Solutions (P/N HWM6-50-Clip). I would have preferred to find the combination hex and Phillips screw head that IBM uses by default, but for the price I'll sacrifice having the hex head and go with the Phillips only. Both the screws and clip nuts are functionally interchangeable with the hardware that originally came with the IBM NetBay 42U rack.
Another possible option would be the Rack Rail Clip Nuts & Phillips Truss Head Rack Screws from Rack Release Systems.
Screw Comparison http://img.skitch.com/20091223-2wipfctbaww74nxxu369k7ub9.jpg