I broke-out the freetextbox.dll control from Scott Watermasysk's .TEXT framework for another project of mine. (Is my understanding correct that this control was written on Linux? It's a very sweet control!) In testing I kept getting an error message "Potentially harmful code was posted ...." or something to that effect. I could NOT figure it out. dotnet.newsgroups via Google to the rescue once again:
Adding <B><PAGES validateRequest="false" />in the web.config did the trick.
Update: Great suggestion from G. Andrew Duthie as a better security practice: turning off Page Validation at the PAGE level and not in the web.config along with filtering out everything except what you want to accept. Also good references from Victor Garcia Aprea on validation. See comments.