Your Stitcher Plus account will be unaffected. You’ll be able to enjoy ad-free listening just as you did before! (You’ll still hear any ads that were placed in the audio feed by the podcasters themselves).
For more information about Midroll’s acquisition of Stitcher, check out our blog: http://www.stitcher.com/blog/?p=1734
2017 update: Please note Stitcher Plus is now Stitcher Premium! Learn more about Stitcher Premium here.