Hi Kai,
ip cameras are a bit of a specialism of mine, so I've got a couple of thoughts..
Although I do use h264 on some of my cameras, I generally just squirt mjpg or jpg to zoneminder. If you have a specific need for icecast (whatever that is!) then the following may not be much use.
Why not use the rpi as a usb webcam host? This idea has been bouncing around for qutie a while, and been used successfully with cheap reflashed routers, guruplugs and others. Simply plug in a bunch of cheap usb webcams and run mjpegstreamer which converts them to a nice mjpeg stream that is read natively by Firefox or Chrome. (And also Zoneminder if you want something recording it remotely, or doing smart things with motion detection)
rpi has zoneminder in its repo which you can run locally too, and record the usb cams to local or nfs storage. (But expect to have to force very low framerates if you do motion detection)
Only real limits are usb or wifi bandwidth.
btw, Y-cams are at the medium-high range of price for ip cameras. Foscam do much cheaper ones at very similar quality levels, and there are also foscam clones flooding the market. Expect to pay ~£30 to £50 for a foscam clone with infra red, 640x480 and pan/tilt. Also edimax have some very low priced ip cameras, some under £50 at 1280x1024. Apart from geekpoints, I can't really think why you wouldn't go for a ready-made ip camera.
