This looks rather good (sadly no iDevice). Looks as if Apple came up with it, if they weren't so silly with the way they applied their skeumorphism.

I always imagined that if you were aiming for the mathematician market (huge market!!1 /s) you could have an app like that except on a tablet you'd use with a pen. Then you'd just write down calculations and OCR would figure out what you are trying to calculate and do the number crunching (where applicable), and intelligently recognise variables and a lot of different mathematical functions (I just don't like the whole process of going back and forth between my TI and my legal pad). That'd be a mathematician's (or just mine) wet dream.