Hi, you know, in terms of development time and cost, the easiest solution would be to have 2 raspberry pi's and 2 cameras. One to record, and one for realtime display. :-) What does a Pi and a camera cost $50? Plus you would have a backup to record with if your main recorder failed. Is $50 worth all the head-scratching required to get just 1 Pi to do it all? Like a few weeks of time for $50? :-)

I also came across something for you to google around for. It is claimed that Gstreamer, which has a Perl module, has the ability to duplex the video feed from one camera. That is what we need, take 1 camera output and split it into 2 identical streams. It dosn't seem it should be that hard, but it eludes us for now. ffmpeg seems to be able to do it, see ffmpeg multiple outputs. I'm going to play with that.

UPDATE0 These 2 links are the most promising leads. Use a kernel module 2 split the /dev/video stream, then use whatever to display 1 stream and record another. v4l2loopback module and splitting /dev/video to 2 streams .

UPDATE1It works! Here is how to split /dev0 into /dev/video1 and /dev/video2

modprobe v4l2loopback devices=2   # as root
 # then                
ffmpeg -f v4l2 -i /dev/video0 -f v4l2 /dev/video1 -f v4l2 /dev/video2
[download]

Now you can watch /dev/video1 in realtime, and record /dev/video2. :-)