Can you tell us about your client computers? Are the macs desktops or laptops? Are the computers on a wired or wireless network or perhaps both? Have you checked to see if wake for network activty is enabled on the clients computers?
Also, assuming the clients are not on static IP, do you know how long your network hands out DHCP leases for?
Tha Macs are iMac 24" desktops in a lab and they're on both wired and wireless networks, and, yes, "wake for network activity" is on. DHCP renews, I think, daily.
ARD 3.4 worked fine when I had a Snow Leopard Admin and Leopard clients, but since I upgraded the admin machine to Lion, there have been connection problem.
Have you tried to do any kind of isolation? IE: ARD computer connect to lan, with a desktop connect to the same lan, same subnet. Open ARD, verify client is listed online. then sleep the computer, can you wake it, or dose that fail?
Mac OS X v10.6: About Wake on Demand has allot of great info about wake for network activty on laptops.