<p>Is it possible to get access to the spell checker that is incorporated in browsers for text areas from Javascript? I would like to be able to control spell checking from withing my code. Most browsers (apart from IE) seem to have some kind of a spell checker built in to them nowadays.</p>

The most access that I know of is disabling or enabling spellchecking on a field:
Inline Disabling of Firefox Spellcheck?
I don't know of a way that you can directly access the spellchecker of a browser via javascript.  If you aren't particular to the spell checker of the browser, there are many open source spell checkers for javascript. Just try googling javascript spell checker.
If you really want to use the browsers spellcheck you might want to create a textbox and set the display to none.  You could then put each word in the textbox and then check to see if it's underlined or not.  I'm not sure of the feasability of this, just a thought.  My suggestion would be to use a javascript spellchecker instead of trying to hack up a way of using the browser's spellchecker.