Hi Ken,
Disk Management grabs the physical disk number from Windows, and Windows assigns numbers based on the order in which it detects disks. That order is BIOS (or disk controller) dependent, and some disk controllers will renumber their disks as you plug disks into different ports.
So it's not random, but it's also not something we have any control over :)
Tentacle Blog:
http://www.tentaclesoftware.com/blog/
WHS Disk Management:
http://www.tentaclesoftware.com/WHSDiskManagement/