Actually, I started working on something like this last week, as a pet project to keep me busy. In essence, what you need is a basic programming language that's geared a little bit more towards mathematics (so you can do little things like 2log(n) instead of 2 * log(n) and actually use π as a constant instead of something like Math.PI), a REPL and a way to graph functions. That's feature equivalence, and that's easy.

The cool part is when you start thinking about how to innovate on top of that. Then you can add things like:

* the ability to display and remember the results of a whole range of calculations on a single screen instead of always having to write stuff down or bring back old calculations, e.g. http://stuff.stdout.be/graphcalc.png (think iPad app)

* the ability for teachers to follow along on their own screen as you're doing calculations, to help out, maybe even allow teachers to take over control (remotely) to show stuff

* save calculations and make little reports, which could serve as homework

* slowly learn general-purpose programming along the way, simply by having the thing based off a language like CoffeeScript instead of Texas Instruments' own half-assed TI-BASIC.

* extensions for physics and chemistry, so it's not just about math

I don't know if it'd really make sense as a startup (tablets are still too expensive and the nice thing about graphing calculators is that they're small and people can't browse the internet on them) but the whole "let's try to reinvent some ancient technology using modern tools" thing is pretty fun.

If anybody else is looking for a weekend hack project, I'd gladly open-source what I have thus far.