You've probably seen this since TV Shows on DVD linked to it, but the New York Times reports that the last three seasons of Mary Tyler Moore will be coming out on DVD, thanks to the recent plug by Oprah.
I think daytime TV clearly has an effect on which non-current shows get released; shows where the cast frequently reunites on these shows. The semi-frequent Happy Days cast reunions probably have helped that show get up to its fourth season on DVD even though the first season didn't sell all that well.
Now the question is whether Fox will release the Mary Tyler Moore spinoffs after they finish with the parent show. Probably not, which may be just as well; there'll be no Oprah plug to help get those back on the schedule.