I did some tests tonight with one of my Linux boxes and a Windows system. I was unable to get video going either way, trying both the latest Windows Live Messenger and Yahoo! clients.
Is this currently broken for everyone? I've had it working in the past, with older versions of Kopete. I'm currently using Kopete 0.12.7, from the Hardy Kubuntu repo.
The Windows system uses a Logitech QuickCam Pro 9000. As soon as I click okay to receive on the Linux box, the Kopete video window starts to pop up then fails. The Windows side then immediately thinks the Linux side closed the video chat. Sending from Linux, with an old Quickcam Express (video shows fine in the Kopete config dialog), the video window never opens on the Windows side.
I ran Kopete from the command line to see if I could get more detail about the failure, but there did not seem to be anything relevant.