I think it is matter of TinyMCE configuration. By default it is disabled (gecko_spellcheck I think). The other way is spellcheck plugin for TinyMCE (http://www.tinymce.com/wiki.php/Plugin3x:spellchecker). By default it is disabled because it requires PHP backend and some additional programs.
At the moment we are working on brand new editor where we will have to implement completely different kind of solutions. This is why we will not be trying to fix this issue in 1.6 version.