I could not find much on the web and it took me a little time to get everything working (especially saving edited content) so I'll share some code, hopefully to save you some time. In apps/views/layouts/application.html.erb I added some boilerplate (with non-editor stuff not shown) to be included with each rendered page:
<head>
<link rel="stylesheet" type="text/css"
href="http://ajax.googleapis.com/ajax/libs/dojo/1.5/dijit/themes/tundra/tundra.css">
<link rel="stylesheet" type="text/css"
href= "http://ajax.googleapis.com/ajax/libs/dojo/1.5/dojox/editor/plugins/resources/css/Save.css">
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/dojo/1.5/dojo/dojo.xd.js"
djConfig="parseOnLoad: true">
</script>
<script type="text/javascript">
dojo.require("dijit.layout.BorderContainer");
dojo.require("dijit.layout.ContentPane");
dojo.require("dijit.Editor");
dojo.require("dojox.editor.plugins.Save");
</script>
</head>
Then in my view I create an inline rich editor using:
<div dojoType="dijit.Editor" id="editor1"
extraPlugins="[{name: 'save', url: 'task'}]">
<%= raw(@task_current.content) %>
</div>
It is very important to use the raw method because Rails 3 automatically makes safe HTML encoded strings for any result returned by <%= ... %>. Also notice the url value of 'task': this sends the content data to the index method of the task_controller. This method checks to see if an incoming request is an XHR POST and if so gets the raw content:
if request.xhr?
if session[:task_id]
task = Task.find(session[:task_id])
task.content=request.raw_post
task.save!
end
end
For brevity I did not show any error handling.

Enjoy!