Uisng ptp or something similar, can I use my Ixus 115 as a v4l device so that I'll be able to stream the live view from it? My idea to stream the video using rtp.
The CHDK PTP extension supports transmitting the live view data from the camera to your PC, but there is currently no v4l driver. My impression is that it shouldn't be terribly hard to write one, but I don't know much about it. One of the "virtual" or loopback drivers might be a useful starting point, see http://linuxtv.org/wiki/index.php/VIVI http://linuxtv.org/wiki/index.php/Loopback
If there is some software you can use to take a region of a window and turn it into a video stream, that could be done using existing software. I know some "virtual webcam" apps like this exist for windows.
If you have a fast system and a high speed link, you can display chdkptp live view over vnc or X.
If the hardware video out on your camera works in rec mode (many recent ones do not) then you could of course do it with regular video hardware.