This is a little script I just finished working on that resizes a reticle on the screen based on distance of the target that’s in the center of the reticle. It also sports changing to a different image based on the tag of the object behind the center of the reticle. It’s just a graphical thing as it doesn’t contain any shooting code or what not.
All you have to do is place the script on an object (more then likely the main camera), drag the reticle textures (I included a basic reticle with an over enemy color) and set the settings. I think I have some pretty good settings for the current reticle set up for you by default.
I’ve uploaded a youtube video for you to see it in action here:
Download it here:
http://calibermengsk.com/games/unity/reticle.zip
I know it’s kind of basic, but should help some people with speeding up the process. It took me a little while to get all of the raycasting to work right for me, but now it does. 
Hope someone finds it useful.
EDIT:
Noticed that it didn’t reposition the reticle on resize (web player to fullscreen as well) so that’s now fixed.