In general, the response from the community was that the right thing to do is to have a finale show, thank all your listeners, and close it down.
I wholeheartedly agree. There is far too much podfade out there (when a show just ends and nobody knows why or when). I think that if we look at main stream media we see that in most cases networks and magazines don't announce the ending of a show or column. In some cases, they do, but in general I think they don't. That works for them, though, because MSM generally has a pretty rigid schedule of shows and it's very easy for a reader or viewer to figure out pretty quick that a show has gone off the air. With podcasting's loosey-goosey scheduling, it's much harder for a listener to figure out if you're gone off the air entirely or are just taking a break.
If and when you have to end a show, tell your listeners that it's over. Then they know they can take you out of their podcatcher and you avoid all the bad karma that surrounds podfade.