I have an app on the iPad called Free graphing calculator.  I have not been using it much but I had my students in the College Prep math class use it today because it allows you to enter in a log with a base other than 10 to graph without going through the change of base formula.  That will make things so much easier teaching logs and natural logs. 
This application does more than just graphing in that it has a calculator, a table window much like the handheld graphing calculators with the x and y values,  the graphing window and the window to enter equations.  It is really nice and combines both calculator and graphing functions.  Just like a TI-82.  I am finding it is quite handy to use without having to juggle a classroom set of graphing calculators.
I am still exploring all the functions it has.  I think I"ll be spending the weekend checking it out more so that I can report back on everything it does.
I think putting enough apps on the iPad to do the job without being addicted to the apps is important.  As a teacher I need to stay at least one step ahead of my students and show them ways the iPad can be effectively used as a tool and not a toy. 
I know many of my students would rather go on the internet to play games rather than doing their work simply because most of my student's parents do not work where a computer is used for its productivity.