Thanks for your comments Brian - I'll answer each:
Curtain mode should work on any system running 10.4 and ARD 3.
KR - that's my understanding. All the systems are running OS X 10.4.8 and see below on ARD version...
Have you upgraded the client (G4) to 3.x? 2.x is the default installation.
KR - good question, I created the client package via Create Client Installer in ARD and manually installed it on the G4. This should have upgraded the ARD client to v3, right? I haven't found a way to verify the ARD client version (suggestions gladly received!) except the recorded version number in com.apple.ARDAgent.plist - which lists 3.000000 as the version, which I'm assuming means v3.0
Is the G4 headless? Curtain mode only works on machines with monitors attached.
KR - that's good to know - the G4 does have a monitor, keyboard, and mouse attached. Along those lines the G4 has the factory installed video card and the LCD display is in 1280x1024 res. I tried 1024x768 res, same result...
Also, in case it makes a difference - I have the Sharing/Firewall on, Services Personal File Sharing, Remote Login - SSH, Apple Remote Desktop, Apple Remote Events, and Network Time allowed. The Macs are in my lab, all on a 100BaseT hub with no firewall or NAT in between each other.