Here's a quick synopsis. Go to System Preferences » Sharing, and enable 'DVD or CD Sharing.' (I think you need to do this on both machines -- the one with the drive and the one without the CD/ROM drive). Then, on the computer that does not have the CD drive on it, go to a Terminal window and enter:
$ defaults write com.apple.NetworkBrowser EnableODiskBrowsing -bool true
$ defaults write com.apple.NetworkBrowser ODSSupported -bool true
Reboot this machine, and when the reboot is finished, you'll see the remote CD/DVD under Devices in the Finder's sidebar. Thereafter, you can remotely mount a CD/DVD from another machine.
[robg adds: The linked hint gives credit for the Terminal commands to this MacRumors forum post. I haven't tested this one yet.]

