Solution

If your webcam lights up fine, you can hear everyone, but you can't see webcam streams, I have the solution for you. After installing the Google Talk plugin, run the following to add an override for your video settings.

sudo echo "O3D_OVERRIDE_RENDER_MODE=2D" >> /opt/google/talkplugin/envvars

Use sudo if necessary. Restart your browser after doing this and check to see if you can see video streams now.