Thanks for your continued feedback on this issue.
As you can see, this issue was created quite some time ago, in fact a VERY long time ago on the scale of Web Browser development.
For a while now Chrome, Safari & Firefox have all had built in spellchecking, so as seen in the comments, this issue has become about providing support for Internet Explorer users.
For those that have been following the development of Internet Explorer 10, you'll know Microsoft has included spell checking as a feature in their recent Developer Previews.
Once released, this will give Internet Explorer users native spell checking functionality, at which point there will no longer be any long term value in having this functionality in Confluence. As such, today we are closing this request.
Confluence Product Manager