well it should do.. but you need CPU power for processing the monitor motion tracking and encoding events... and an the raspberry isn't capable for that.
You also don't have an PCI-slot so you only can use USB or IP-camera's.
So I think it's an no-go!
Zoneminder @Ubuntu 11 server, ZM 1.25.0 , FFMPEG, 4 Analoge cameras, 1.6ghz P4mobile, 2GB ram, 60GB-HDD. I also have an (homeseer) domotica system up-and-running with touchscreens,light controllers,weather forecast etc.
Still, that's just outputting a pre-formatted video file. It'll be able to run clients (i.e. chrome/firefox/?) and output them to the display just fine but I doubt it could do real-time transcoding or even analysis of frames. Maybe if you slimmed down the build on it and removed the X11 display, went with DirectFB or something. Streaming directly from your endpoint to a display looks more like a realistic use for RasPi.
That is awesome that they're making plans for PoE.