Here is my definition of MMP, given so that the rest makes sense:

A MMP game is a Virtual World whose purpose is interactive entertainment and intraplayer socialization. It will have an asynchronous populaton (aka: total subscriptions or accounts) in the 1000'S and a synchronous population (ie: players that play with each other) in the 100's.


Not "strictly" true and depends on what you mean by "one server".



THE SOFTWARE MODEL:
The canonical multi-tier software model involves Logic, Presentation, and Data. In a SP game, this is all handled on the same computer. In a MMP game, the Presentation is the client computer; Logic and Data are handled serverside.

In a MMP, the data is a MySql DataBase server (or your DB engine of choice) populated with your games tables and stats. Normally this will run on a unique computer separate from Logic. The Logic is the games dynamics. It can be physics, combat, chat, trade, etc... it is the rules that govern the game... the DataBase server just provides the numbers for these rules. Both are networked serverside so their connection is in the gigabit range.



THE "MASSIVE" SINGLE COMPUTER SERVERSIDE:
If one server means one physical computer, then you would have to run Logic and DB on that single computer. With this deployment -- and with programming taking into account this one server deployment -- my guess (please note: just an educated guess) is that you would be able to get about 100 players (not subscriptions) per computer. This fulfills MY definition of massively multiplayer but just barely.



THE MASSIVE MULTI-COMPUTER SERVERSIDE:
However, if you put the Logic components onto one physical computer and the DataBase onto another physical computer, then you can optimize each computer accordingly: CPU is a priority for the Logic server; HDD is the priority for the DB server. In this case it is my guess (again: educated guess) that you will be able to serve on the order of 1000 players this way. This is not only due to the optimized computer hardware, but moreso on the optimized programmed software that only takes it's specific role (presentation, data, or logic) into account. Note that these same two computers could have served only 100's under the single server model.



THE (not so) MASSIVE CONCLUSION:
Thus in this "back of the envelope" arguement both deployments fulfill the criteria for being MMP per zone, but separating Logic from DB means that you can really guarantee that this will be so since each "zone" has the capacity for thousands of accounts and hundreds of players at the same time.