I'm an engineering major, and love love love the TI-89.
I agree that it's too big and bulky, and should be an iPhone app.
This being 2011, I feel that it shouldn't be too daunting of a task for such a successful company like Texas Instrument. I can live without my flying cars, but a TI-89 in your pocket can't be impossible.
I've tried to find good calculator apps before. pi^3 is good, but doesn't have everything. The Wolfram Alpha app is cool, but the interface is nothing compared to the TI-89. Not only that, Wolfram requires the internet, which is generally a big no-no come Calc 2 test day. But getting a TI-89 app, setting your phone to airplane mode, and letting the teacher watch your every move to make sure you're not multitasking to Evernote or Wolfram with that iPhone 4, and you might just be in the clear.
I advise all people who agree that TI should create an application for the iPhone/iTouch should let your voice be heard by TI. Let's all write em a sternly written email or something. Who's with me?