A little more information, maybe helpful to somebody!
I migrated 4 G4 mac minis to core2 duo minis a few weeks ago.
First machine, when it came up with updates it offered the ARD 3.5.1 Client so I downloaded the dmg for that and installed it. While I was at it, I (stupidly!) downloaded the 3.5 admin dmg and installed it on my machine. This new mini then entered a weird state on my ARD -- still shows as a G4 processor, and still shows 10.5.8 as the OS. The only thing that I can do with this machines is Control and Observe it. But it's shown as "Offline" so I cannot push software, copy files, run reports, etc.
Next I did the other 3 machines. I left them at Client 3.4 (came over with Setup Assistant) in hopes that they wouldn't cause me problems. On my 3.5 Admin, they behave exactly as the first one -- the Admin is sure that the computers aren't there, but if I Observe/Control them they come right up.
My boss (heeding my warning) left his Admin at 3.4. These four machines did the same thing for him, but he was able to fix it by deleting them and then re-adding them by IP address. I tried that, and they reappeared in the same broken state as before for me at 3.5 Admin...
I noticed yesterday that my one machine that is at 10.6.8 has entered the same unreachable-only-to-me state.
Ugliest effect... I have this workflow where I periodically run a report on every machine which shows up on my scanner. A few machines will fail for various and sundry reasons, but if I would let it run for awhile and then hit the "stop" button it would bring up the report and I could export it to a spreadsheet and there would be blank lines on the few machines that didn't answer. Now, however, when I "stop" the report it just aborts and gives me NOTHING!!!! AAAARRRRGGGGHHHH!!!!
I have the same question as rcmgk -- how do I delete the software so that it will let me re-install 3.4? (I have the ARD 3 box with install disks, and the 3.4 Admin dmg.) Is there any way to save my entire setup of lists and categorization that I have painstakenly built up over use?
I suppose this is just one more step of Apple walking away from the enterprise. We are already shopping for linux rack-mount servers to replace our aging Xserves.