One of the new features of the 3.5 upgrade is integration with the iespell plugin - only works with internet explorer.
If you want to download the plugin, you can go to http://www.iespell.com
, and when you see your post box, it will have a little abc/checkmark in the upper right that will check your spelling.
Another option is to download the google toolbar. It has spell-check that works across several internet sites, but I have had better luck in IE than Firefox.
For Firefox users, there are extensions that you can download and install for spell-check.