What steps will reproduce the problem?
1. install several capture devices (each opening a different /dev/video*), or 
no capture device at all
2. make sure that the v4l2loopback driver creates a loopback device different 
from /dev/video1
3. try using v4lsink

What is the expected output? What do you see instead?

/dev/video1 is hardcoded, so v4lsink will fail to run if this file is not 
present or not a v4l2loopback device.

i would like to be able to tell v4lsink which device to use. so i can adjust it 
to my system without having to recompile (think usb-cameras...)


What version of the product are you using? On what operating system?

mercurial checkout on debian squeeze/sid


Please provide any additional information below.

attached is a patch that adds a "device" property to the V4l2SinkLoopback 
element.
it defaults to /dev/video1 

example usage:
$ v4lsink 'videotestsrc ! ffmpegcolorspace ! V4l2SinkLoopback 
device=/dev/video0' 


note that this is the first time that i have written any vala code.
so please check the code for any problems :-)