I wanted to use a Rich Text Editor for a text area inside an update panel.
I found this post: http://www.queness.com/post/212/10-jquery-and-non-jquery-javascript-ric
One thing that I ran into when putting TinyMCE in an update panel was that I had to do all sorts of tricks to make it work right. Keep in mind how the update panel works, when you want to refresh it, it completely replaces the DOM inside of it, including the TinyMCE.
The workaround that I used was to remove the TinyMCE from the update panel and wrap anything else that needed to dynamically refresh in an update panel. I would then add data to TinyMCE via the javascript API it provides if I needed the TinyMCE content to be dynamic.