Well I for one would like spell and grammar check in an email application, I would quite appreciate it.
I'm not the best speller out there and it helps me get my learn on, and keeps me from showing everyone just how bad I really am.
Maybe even optionally for texting but honestly I'd just find that annoying. Often times I'm purposely shortening words because I'm working on a 160 char limit and have a lot to say. And in texts I'm far less insistent on having stuff spelled right anyway.
But, that's just Phases. Indeed I should, and do, take punks suggestion and try to improve my spelling inadequacies, but in the meantime....
:P