The built-in automatic activation for Flash seems to be working fairly well, but most of the video I watch in my browser on the machine I'm running Caffeine on is Netflix, thanks to Pipelight making Silverlight video accessible. It would be nice to have a checkbox for Silverlight/Pipelight video alongside the one for Flash.
Right now I've got it set to disable the screensaver when Chromium is running, but I'm not sure that's working properly, and even if it did, it would disable it even if there's no video running.