It would seem that questions about the use of OpenCV should be on topic, because they are usually questions about computer vision algorithms in disguise. There are plenty of such questions on StackOverflow, and I think that they belong here.
I have looked at the last few openCV questions. Most of them seem to be about compiling or use of functions, which would be off-topic for us.
Among the on-topic questions, two are rather low quality (here and here), though the third one might be interesting.
Personally, I feel that that third question should be on-topic for DSP, even if the user needs a solution in a specific language.
1$\begingroup$ The third one was asked here dsp.stackexchange.com/questions/401/… $\endgroup$ Nov 23, 2011 at 19:55
$\begingroup$ I agree. A DSP related programming question should be on-topic. May be we should migrate them to SO once answered (periodic cleanup?). I think people using openCV are concentrated here than on SO. Are all DSP/SE members also members of SO? $\endgroup$ Oct 26, 2012 at 18:27
Programming wih OpenCV is off-topic. That's purely SO. Questions about how OpenCV algorithms work are on topic. In general, OpenCV questions will be of type Which OpenCV function can I use to achieve effect X?. That's a StackOverflow question. OpenCV is an API, not a particular computer vision algorithm or idea, so most questions will be programming-based.
2$\begingroup$ While I completely agree I want to point out that ORB descriptor was developed specifically for OpenCV, to get patent-free descriptor compliant with OpenCV license. So it could be considered as "OpenCV algorithm" $\endgroup$ Nov 20, 2011 at 12:29