In our introductory CS course at UC Berkeley, CS61A (which is based on SICP but taught mainly in Python), we use http://pythontutor.com to visualize environment diagrams for our programs line-by-line.
Hi Kronopath, the app isn't on the App Store just yet. It was sent for review, meaning it could take another week or so before I actually know whether it will be approved for sale or not.