I’ve been having a look at some iPhone apps that offer similar functions to the graphic calculators we use here at school (the Casio fx-9860 and its variants), and there are some good ones around.
There are lots of free apps that do various useful things, but the best overall one (in terms of ease of use and functions) that I've looked at so far is called Graphing Calculator.
It costs AUS$2.49, so it’s cheaper than chicken and chips, and much cheaper than a “real” graphic calculator.
(I’m glad I’m not teaching a pre-tertiary at the moment…. How many times would hell have to freeze over before the TQA would allow students to use an iPhone in an external exam?)