It's not. Mojang is foolishly pushing forward for its own content, and has decided against trying to add community based work for-the-moment as a whole. I don't foresee a change in this policy...
Since Jeb took over Mojang, there has been a bit of a development drift from small aspects being added into a large sand-box (Notch style) to more organized rewrites and story driven content to add theme (Jeb style). For a good period of time, this was all fine and good. But since Jeb and the Mojang team have taken it upon themselves to flesh out Minecraft into more of an adventure RPG world, rather than an open sand box of creativity, they have ignored the community contributions and begun doing as they like in result...
It was Notch's haphazard laziness that prevented us from getting a Mod API before the official Minecraft release. Now, it appears that Jeb and Dinnerbone have changed objectives entirely, and would rather just continuously add content in a DLC like manner than focus on creating official tools for community creations. Which is a shame, since I think a valuable, educational, golden opportunity could have been seized had they allowed more official support for Minecraft modding...
Maybe not so much. I think Dinnerbone said somewhere on his Twitter feed that most of the newer changes would bring them a few steps closer to the Mod API, but it would cause modders trouble in the meantime.
- 1.2: Increase of world height to 256. Gives some biodiversity and environmental mods more space to develop.
- 1.3: Merger of SSP and SMP. This did throw quite a big problem (some mods like RedPower never truly recovered) but modders have to maintain 1 version instead of 2 separate but similar versions.
- 1.4: Not a lot of changes (except maybe some NBT data improvements), and there was little trouble for modders upgrading to 1.4 from 1.3.
- 1.5: Texture pack changes, now more modular and also ensures some future texture-compatibility for texture pack makers.
- 1.6: New launcher helps setting up multiple instances for mod users, providing convenience.
- 1.7 (upcoming): Depreciation of IDs removes many bogus bug reports, and cleaning up of 500 000 lines of code definitely makes Minecraft smoother and improves the quality of a hypothetical mod API. Biome mods are possibly less "hacky", the sound engine may let modders add better sounds than before. EDIT: Yes, and packet rework too, that will be a trouble. (Thanks RawCode)
All the odd number changes (1.3, 1.5, 1.7) posed/will pose a big problem to modders, but they are bringing us closer to the mod API (whenever that is).