There are already Minecraft mods, of course, but these plans involve a full source code release.
Notch outlines the plan for officially supported mods here, including licensing details. The rules are pretty simple; mods must be playable only by those who’ve paid for Minecraft, mods can’t be sold for profit without a specific licensing deal from Mojang, “malicious” stuff is out and Mojang retain the right to implement the mod idea in Minecraft themselves.
Initially, the plan included a “sign-up cost” (undefined, but said to not be “prohibitively expensive”) for players who wished to register as “mod developers.” Only one would have been required per mod team, but the internet reacted … well, let’s say ‘poorly’ to this idea. In response, Notch quickly axed the sign-up fee.