Hi,
Thanks for reporting this problem, I was not aware of it, but after looking into it it seems to be a known issue. We really need Microsoft to fix this so you can help by voting for this work item on codeplex
http://ajaxcontroltoolkit.codeplex.com/workitem/26772
In the meantime, the only solution I can think of is to add logic to degrade to a plain textarea if running in medium trust. I will do that before the next release of mojoPortal.
Thanks,
Joe