I have contacted our RadControls teams about this issue, and they informed me that it is a known issue, fixed for the Q1 2009 suite (which should be released in a few days). Since Sitefinity is using Q3 2008, this problem is reproducible on your side.
We are going to release Sitefinity 3.6SP1 in the end of March, which will include the latest Q1 2009 release of the RadControls. That said, an option for you is to wait for our service pack.
In the meantime, you could try to set ContentFilters="None" property in the RadEditor declaration. If you are using Sitefinity 3.6, you should map the control templates where the RadEditor resides, and make the changes in them.
the Telerik team