A friend of a friend wrote this: http://mathtouchapp.com/ (He doesn't know I'm linking it here).
The app throws away the calculator metaphor and starts from scratch. Instead, the author uses the back-of-a-napkin as the metaphor.
You start with a blank page and add systems of equations. You can visually link variables together. You can insert values along with its unit of measurement and numeric precision. You can feed results to graphs.
At $10, this is cheaper than getting the Nspire if you already have an iOS device. But of course, you can't use it when taking the SAT. And it would be cool if you can export it to Wolfram's computational data format and trade formula libraries.
You need to actually understand the math instead of just punching the buttons. Then again, isn't that what Sal Khan's videos are for?