I was doing a clean install of Snow Leopard on an older MBP, and got the message “Error: Could not unmount disk.” Somewhat confused at first - I tried it a couple times, restarted the machine, etc. Still no go.

For some reason the hard drive had some open files or applications - which would cause the error. The solution is not obvious, and I just happened to randomly try this and it worked!

-Boot to your into the Snow Leopard installer.
-Open Disk Utilities
-Select the “Macintosh HD” partition, and press the “Unmount” button. (You’ll get an error asking you to make sure all files and applications are closed)
-Select the DVD-Drive (not the disk) (mine was MATSHITA DVD-RXXXXX), now press the “Eject” button in the toolbar. (You’ll get the same error as above)
-Select the “Macintosh HD” partition again, and press the “Unmount” button. This time, it will successfully unmount.

Now you can continue to erase the Macintosh HD partition, and do a clean install of Snow.

Hopefully this works for more people than just me.