Don't use a Ti-89 in highschool, you will just spoil yourself - in other words you won't learn anything. Also, at my college generally no math class allows calculators, only classes where math isnt the main subject - and even in those math isnt that intensive. Ti-89 is a great shortcut if you already know the math, otherwise your just cheating yourself.
Also who the hell would go through so much trouble to put the 89 in another casing? You said yourself that:
"It's not so much that I plan to cheat but that I don't want to shell out a lot of money for a graphing calculator now then two years down the road finally be allowed to use the one I want (TI-89) in college."
Wouldnt buying a calculator, gutting it, and placing ur 89 inside be more expensive then just picking up a used 83 and selling your 89?
I guarantee you wont need a Ti-89 in college, if you need to do math you can always use matlab, and during tests either no calculators or just no 89's