TinyMCE 2.0.5, including the new spell checker, is in WP trunk. We auditioned this on WordPress.com for awhile and received lots of positive feedback. Over the history of WordPress, the number one most requested feature is WYSIWYG editing. We added that in 2.0 when we incorporated TinyMCE. With that down, the number one request became spell checking. A spell checker is a make or break feature for many people. So, here it is.
(This post not spell checked because I am a hard core. I dare you to find a single mispeling on this page.)