How useful could the Arduino Yun be in computer vision in a robotics project?
It probably has enough memory and speed to run basic object recognition software, but I there's a better solution, and that's to do your image capture and your object recognition with another device, and use the Arduino to act on the incoming data in some way. That would give you enough juice to do object tracking or searching. I plan to do this for my copter gimbal hopefully to be able to track objects on the ground, like criminals running from the cops for example.

This product will work great for that. Since you need a camera anyway, I think this is a pretty good deal... http://petapixel.com/2013/09/01/pixy-low-cost-camera-recognizes-follows-objects-color/

Can be purchased here: http://www.adafruit.com/products/1906

AdaFruit has tutorials to help you get started.  (Computer-vision doesn't exist as a tag right now and I can't create it, but I tried, sorry)