OK! I received the drive caddy. The blank drive caddy had p/n 4P124. I am not sure if that is a Dell p/n but the part arrived from http://www.parts-people.com. I went out and got a 120GB 5400RPM 2.5inch laptop internal IDE drive.
Now, I built my bootable USB stick to install an OS on the new drive which has detected beautifully and dual-boot this puppy. For instructions to do this, see:
http://owsug.ca/blogs/brad/archive/2008/10/01/Making-a-USB-stick-bootable-for-Vista-or-Server-2008.aspx
Success!
Incidentally, in the boot sequence and as detected devices, Modular Bay drive shows up. I have no idea what Dell was thinking that this part did not exist... I installed my second OS and now have a dual boot configuration 1 OS on each drive.
Experiment complete. I now gained way more functionality from my laptop with a second internal drive!