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
I think you want to look at this posting: How to make TinyMCE work inside an UpdatePanel?
Make sure to register you init function with the scriptmanager
init
scriptmanager
ScriptManager.RegisterStartupScript(this.Page, this.Page.GetType(), mce.ClientID, "pageLoad();", true);