They have been delaying, but they have also hired several of the developers from CraftBukkit, which did create a plugin API for Minecraft. Their experience with CraftBukkit showed that they had created the API too soon. The code is messy and not amenable to mods.
The 1.8 update, currently in development, has added features, but it has mostly focused on refactoring and cleaning code. For example, they recently rewrote the entire block id system, removing numerical ids and instead using namespaced identifiers. This will allow mods to coexist rather than try to claim the same block ids.