In answer to some of the other questions about whether hiding the message when it hasn't been moved or permenantly deleted will mess other things up, the answer unfortunatly is that it will. The message count in the bottom and also the retrieving of new messages to maintain the proper number of messages in the list would both be borked by this.
This is a pretty critical issue, and one that I don't think is easily addressed. Therfore, I'm changing my proposal such that I now believe that the default style for deleted messages should not be hidden, but should instead be greyed or struck-out.
The hiding of messages seemed like a good idea because it preserved the status-quo, visually speaking, but it already required a couple of non-ideal work-arounds (read: minor cludge), and I don't think fiddling with the messagecount to account for stylesheets is a good idea.
So in conclusion, if you do not have a Trash folder defined, deleted messages will be flagged for deletion and will appear in that folder with a visible style such as greyed or line-through. The whole make delete-flagged messages invisble is a bad idea, I now believe.
-Charles