I don't think the Reagan one was a poke at his disease but his "reputation" of being unaware of what was going on around him while in office. I think the reputation was undeserved, but I didn't take the joke as offensive.
What about Heisenberg's Chicken?
You can't measure precisly when it crossed the road without changing the result somewhat.
Doesn't really answer the question of why though.
EDIT: Heh, somehow I missed your Heisenberg one in the list. oops.
[ January 23, 2003, 19:15: Message edited by: geoschmo ]