Data > Convert > to Rich Text
(and the matching convert command in the Applescript library)
are, of course, very useful,
- Italicisation is lost,
spaces are stripped down to a single minimum paragraph break.
It would be very helpful if these limitations could be overcome. In the meanwhile I am having to resort to applescripting TextEdit, which gives a slightly more polished HTML->RTF conversion, but is a little cumbersome.