This is from a years ago - but it has some interesting examples of the "First Use Doctrine" - not just for fabric:
http://answers.google.com/answers/threadview?id=398157
Obviously, you have to use some common sense.
Your should NOT state anywhere that it is an authentic Yankees product.
You're using authentic licensed fabric - meaning you didn't go to Spoonflower and print out yards of the logo (although SF wouldn't allow you), or used photo fabric where you printed out images and logos.
Now, if you're mass producing 1000's of these in China and trying to sell them on 161st St outside Yankee Stadium ;-) , it's a much bigger issue than if you're making stuff to sell to raise money for your Little League team.
But check out the suggestions for labeling the product.