There is no official sdk as of now, and I probably won't use programs written with it when it comes out, because they will most likely cost money and be distributed through itunes.
Preforming a "Jailbreak" and using native apps can in no way void your warranty, however, becuase it simply changes a unix property of your drive, which can easily be undone through itunes at any time. They have know way of knowing you did anything! Unlocking, on the other hand, or messing with the baseband and such, can and does break your terms of service (if you use at&t in the us), and I believe it also voids your warranty.
I don't have an iPhone to unlock, but I don't think I would care about the warranty, because it would just ware out in one year anyway.
Although, you seem to be right on your first point, it wouldn't work at all with the text and such. I just promised someone on another forum I would ask about getting a decent rts.