This has been so ridiculously helpful, and the tutorial is so simple that I was able to follow it with absolutely no prior experience with photoshop. I know this thread is ages old, but this trick is great. Thanks so much!
The other "cheating" way to brighten eyes is to use the dodge tool at about 20-40% hardness making the size just big enough to cover the iris without touching the pupils. It takes some playing around with but the results can be nice.
I am going to have to try this in Corel Paintshop Pro as that is what I use, I will let you know how it goes!