While trying to iChat using Leopard to a system running Tiger, I ran into a problems that I never had using OS X 10.4 before: bad video quality to downright refusing to connect.
With a little research, I ran across this article and that was enough to resolve the problem.
Here’s how to get iChat working on OS X 10.5
…if you’re running Parallels.
See, turns out that Parallels, I’m using 3.0 Build 5582 (Dec 5, 2007), appears to be running some services, even when the virtual machine is active, that gets in the way of iChat.
Get out of iChat.
Go to Apple / System Preferences…, select Network, and click on Parallels NAT and change the Configure drop down to Off; then go to Parallels Host-Guest an change the Configure drop down to Off. Press Apply.
Get back into iChat and try again. For me, it instantly fixed the problem.