I've just setup a test install of zoneminder with a Foscam IP camera. The camera can deliver 640x480 at 15fps in mjpeg and 8fps in jpeg. Zoneminder will process either of those just fine.
At 15fps though zoneminder uses too much CPU for motion detection. Ideally I'd like to do motion detection at 5fps and store events at 15fps. That isn't possible because to set the "Maximum FPS" setting I have to use jpeg and am then limited to 8fps. It seems to me this would be a simple matter of skipping 2 out of 3 frames of the mjpeg stream for motion detection. Has anyone tried to do something like that? I may have a look at hacking that together.