I reported this several months ago - in fact more than two updates ago. Can't remember which update introduced the problem but I used to be able to drag my video chat window over onto a second monitor so that I could work on documents etc on the main monitor while still viewing the person to whom I was communicating.
Now the window goes completely black whenever it is placed in the second monitor. I have discovered that if I drop it half way, the half on the main monitor stays live and so does the other person's video, but the wee window showing my video (if it is in the left corner, and on the second monitor) turns black.
As I said before, this used to work perfectly and, other programmes like iChat and other video apps, have no problems with this. I am using an Apple Mac G4 with OS X 10.4.11
Would appreciate acknowledgement that you are aware of this problem. I have seen only one other forum topic on this subject but there was no response or replies.