There are a couple of things that make TI calculators different from your average hackable smartphone or Raspberry Pi device.
First, they have terrible specs. The TI-NSpire, which is the creme-de-la-creme of these calculators has 20 MB of RAM. Compare to a recent Galaxy S3 smartphone - 2 GB, or even a $35 Raspberry Pi - 512 MB. The CPU is also woeful in spec, as is the flash, etc. They're also locked down to their dumbed down operating system which is extraordinarily limited, even when you consider the lack of the device's hardware prowess.
Second is the fact that these TI calculators are allowed on the SAT, ACT, and other standardized tests. This in my opinion is the reason for the awful specs. That they are allowed is precisely because the calculators are limited. It would be much easier to put a whole bunch of cheating software on a Galaxy S3. Heck, I could see apps like that in the Android marketplace for sale for $9.95! Or imagine - you could "ask an expert" during the test and have the answer transmitted to you over the smartphone's 4G coverage. Doesn't it make sense that the TI calculators have no built-in-networking, not even Bluetooth?
TI has to balance the fact that people want to cheat their way through these tests or math class and yet give them a nice calculator, one that can aid students in relieving the drudgery of basic arithmetic and maybe even have features that make people's lives easier. But again, this cannot come at the expense of having a platform that's ready for cheaters.
So in my opinion, this concept of putting Linux on the TI N-Spire is probably not a good idea, for a number of reasons. First, TI will likely try their darndest to prevent the calculators from being loaded up with a custom OS that could then be loaded up with cheatware. Moreover, if cheatware became easy to load, the people that run the SAT and ACT test would look to disqualify the TI from being used on these tests. This would then hurt the people that are honestly looking to use the calculator as intended on those exams.
If you're looking to hack on a piece of hardware, buy a Raspberry Pi, load a custom ROM on your smartphone. You're not doing anyone any favors by hacking Linux to run on the TI calculators except cheaters, and even that would only last a short while.