There appears to be a few changes when it comes to the spellchecker, which may not be easy for people to find so I thought I'd put them here.
First, the AJAX spellchecker now requires it's own httpHandler in the web.config in order to function properly. If you don't have the handler setup and you load an editor with the spellchecker tool, it will fire an error (albeit a rather descriptive one!). So, you'll need to add this to the web.config:
<add verb="*" validate="false" path="Telerik.Web.UI.SpellCheckHandler.axd" type="Telerik.Web.UI.SpellCheckHandler, Telerik.Web.UI"/>
The next change is the location of the dictionaries (en-EN.tdf, for example). The old editor stored these in the ~/RadControls/Spell/TDF/ folder. This meant that you had to explicitly allow the Network Service account write permissions on that folder / file in order to add custom words to the dictionary. Now, these files are stored in the App_Data folder, in a sub-directory called RadSpell (so the path is ~/App_Data/RadSpell). I think this means you don't need to explicitly set permissions - but I could be wrong on that one (haven't had a chance to test it yet). In any case, the AJAX spellchecker needs to have the dictionary(ies) in that location in order to function.
FYI, the official help file is located here:http://www.telerik.com/help/radcontrols/prometheus/EnableSpellCheck.html