Honest to god - I thought "I don't remember writing this post??" I'm having the exact same problem...but, it's not just my white t-shirts and it's not just t-shirts anymore. All of my t-shirts are getting discolored under the arms (and some almost brand new) and I just pulled an expensive sweater out of my closet and saw the same thing!! I bought the new "Shout" set in stain formula and hope that will help. I also bought the new prescription strength anti-persperant. I hope that helps.
If anyone has any tips on removing the stains/discoloration, I'd love to hear them.