Given that most modern browsers have built in check of some sort (except maybe IE, I’d have to use it to know…), it’s probably best that it not be a part of the app and more than it is for WordPress. I’m sure someone could make a plugin.
All modern browsers except IE can do spellchecking. IE can do spellchecking if you install ieSpell.
That is really sad that IE 8 doesn’t have a spellcheck.
Thank you for your replies, I didn’t think about a spell check on the browser side. This should do for now. 
I’m currently using a modified version of Brandon Aaron’s spell checker on the client side, and have all but disabled it on all of my sites.
1) Some browsers do spell checking as standard, so it was overkill.
2) All browsers which do it as standard do so through an addon (yes, even the Mozilla suite).
3) Different people want the spell checker to work different ways – and it was a nuisance for some, and just plain old slow for others.
4) For subject specific forums, words not in the dictionary started to become a pain. I run my little brothers WoW guild’s website for example, and it annoyed some of them senseless.
5) The client side version isn’t the fastest
6) The server side implementation just adds another step to the posting process.
7) You complicate issues with having an “add to dictionary” or “ignore this word” feature, both of which are kind of standard in spell checkers.
It’s a nice idea, and i liked it once i wrote the plugin… but it just became too much hassle very quickly.
