iPhone iOS UIImage how to detect a "laser pointer" dot on a camera feed?
OpenCV is compilable for iOS. While it may not be the most efficient it gives you the option of porting the algorithm. I have done a similar marker tracking problem using the ConDensation algorithm. Look up marker tracking with OpenCV. It is a VERY large research area and the exact algorithm you want depends completely on your application. If I remember correctly there are around 3000 reported image processing techniques - to pick a good one is a real art!


  One solution would be to sample the color of pixels within a certain
  radius of the center and detect a blob bright color. Can anyone
  suggest an algorithm for this activity?


By the way, this is the basic idea behind what is called a particle filter (of which Condensation is one method). Well done you figured out the basic idea by your self!


  Another approach would be to keep track of the average position of the
  dot over the last few frames, thus reducing the guesstimate radius. If
  there's no dot within a pre-defined region, the search region may be
  expanded.


This is called state persistance and can be modelled in various ways. The condensation algorithm uses a stochastic approach not dissimilar to a plain old Kalman filter. 


  Is there an efficient way of comparing multiple small images to one
  another to understand if their silhouettes match?


This one is a little more difficult. You could try template matching but I don't know how well it will perform on iOS (very heavy calculation and iOS camera is not well suited to it).