This is interesting because I just noticed the same problem with the editor appearing to not close <dt> tags. After editing some content, all my </dt> tags were gone!
You said: "That's the IE behaviour" and, indeed, when I use the editor in Firefox it does seem to behave differently (the </dt> tags did not get stripped).
Would you classify this as a bug that you can fix in a later version?
You also suggested using the Editor.XHTML property to work around the problem. This is not a satisfactory workaround for me because I was data binding to the Editor.Text field which is read/write. I can't bind to the Editor.XHTML property without writing extra code because it is read-only.
Any chance you'll make the Editor.XHTML property read/write in a later version?
Thanks.
(I'm only using the trial version right now while I evaluate which editor to use on my site.)