Posted for "Douglas W. Johnson, MD" <Lancair1@bellsouth.net>:

Yup, happened to me too (see the list archives in 1998) in my IVP climbing
out south of Chicago after leaving OSH--drop in MP climbing through rain and
then thick clouds at FL180.  No trace of ice on airframe. Fortunately the
alt air door came open after I declared a problem and was descending through
15000ft.   Beware heavy rain followed by a climb through the freezing level.
Glad all worked out well for you, and thanks for the detailed scenario.
Doug Johnson