It's for every browser as far as I know, it's always been functional for every browser for as long as I can remember. A little red squiggly line shows up under words. You might want to check your profile depending on your account profile, you may have it disabled.
As far as IE itself goes, I stopped using it almost 5 years ago when I switched to Firefox because it was a better browser. I now currently use Chrome, again because it's a better browser.
I heard a great comment on the Tekzilla webcast that said that they were thankful that recent statistics have shown that the number of users that still use IE6 have been reduced to 1% of web traffic..
I was floored by this for one simple reason. IE6 has been obsolete for like 10 years. The only reason Microsoft still releases patches for it is because... People keep using the damn thing!
FireFox and Chrome have currently intelligent release systems in place, Chrome started it. I'm not sure if IE will follow.
Far as I'm concerned I'll never use it again, even the few sites that used to require IE no longer do, soon as I find a way to actually remove it from my OS I will, it's 'sticky' though.