I put up a post on the Blog here about Miller's contract, the cap and the tax (
http://www.phillyarena.com) To make a long story short, if they trade Miller and take back equal salary, they will be up against the tax this summer, so probably won't be willing to use the MLE. In fact, if they resign him to anything over 6M(MLE range) they still will be at the tax if the use the MLE.
This leads me to believe they will just let Miller walk if they think they can get a better player back using the MLE than what they can get back for Miller in a trade. Ideally, you'd want them to make a trade and then use the MLE, but that assumes they will go over the tax or can move Sam or Evans- which are all unlikely scenarios.