The documentation in the online help for the Telerik MOSS Editor doesn't explain how to enable the spell checker, which you'll want to do since the MOSS spell check will check the HTML embedded in the editor resulting in unwanted results.
There are two version of the spell checker, an inline AJAX version, and a dialog-box based version.
The AJAX version is inline with the text, and is similar to the feel of GMail's spell checker.
The dialog-box based spell checker is more like Word's.
To enable the Telerik editor's spell checker, enter one of the following lines into the ToolsFile.xml file located at %PROGRAMFILES%\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\188.8.131.52__1f131a624888eeed\RadControls\Editor:
<tool name="SpellCheck"/> for the dialog-box based spell checker
<tool name="AjaxSpellCheck"/> for the Ajax spell checker
This is the same as for the standard Telerik RadEditor for ASP.Net, whose documentation can be found here: http://www.telerik.com/help/aspnet/editor/CustomizingRadSpell.html