Petr, the current spell checker plugin we use is general plugin without PHP implementation, I found a php spell checker with php backend developed by tinymce team: http://tinymce.moxiecode.com/download.php
Do you mind I remove the old one, replace it with php spell checker?
It provides three engines: google, PSpell and PSpell shell.
PSpell needs command line aspell installed. And PspellShell need to compiled into PHP, so I think we should use Google engine by default, do you agree?
Feel free to reopen if you find bugs of this.