I was checking some rounds with my caliper today and noted that the 9mm casing has an outside diameter that is smaller at the mouth where the bullet is crimped in place (0.95mm) than at the base by the lip (0.98mm).  I checked all the other ammo types I have and none of them, including both semi-auto pistol and revolver ammo, shows any casing taper whatsoever--they are cylindrical.  Does anyone have any idea why 9mm is this way and are there other calibers out there that are also tapered?

The 9mm was designed that way. It was actually first made by using an un-necked 7.65 Parabellum (.30 Luger) case after the German military (the Navy first, I think) expressed concern over the potential stopping power of the smaller round in negotiations for pistol sales. The taper is probably an aid in functioning, but exactly to what extent is arguable since there are many other reliable pistol cartridges that do not have such. The Russian 7.62x39 case is also tapered, and in this instance is undoubtedly a factor in the reliable feeding and extraction of this round.