We weren't allowed to use calculators in any of my calculus classes.
However, if you are an engineering major I would higly recommend the TI-89. It is incredibly user friendly and does everything you could possibly want. It is FANTASTIC to have around for those tests involving 4+ multiple linear equations with umpteen unknowns. When everyone else is rewriting them for x then putting x in and solving for y, or practicing their matrix solving skills....blah blah blah....you are done. The other thing, and this is silly, is that it does unit conversions. Now, this isn't particularly hard, it's just that you don't have to hunt down the conversion chart(s) when you get some weird problem with furlongs/dynes/lb-ft or something. Last fall in my thermo class we had a TON of problems that really just involved integration and then the correct simplification to be solved. With TI89 it's just a matter of punching in the equation you have and it'll integrate and simplify for you. it really is an awesome calculator. The data manipulations and statistics functions are also great. If you are planning to take a numerical methods class it will be quite useful for in-class exams that be possible without MATLAB or some similar software. Of course, you still need to learn the basics sans 89 so you wouldn't be completely reliant on it.
One thing, though, if you don't take advantage of the functions available on the TI83 you aren't likely to do so on the TI89. So many people don't know what their calculators can do, regardless of model. Make sure you really look through the TI83 manual and see if you have the features you want already, but just aren't using them.
I use my TI89 nearly every day in grad school. Towards the end of undergrad I found that the TI89 did things that others had to use MATLAB or MAPLE for.
