More news about the newly named 'Workbench' Mod API is showing up, much to the delight of Modders everywhere. Nathan Adams, a Mojang developer, said in a tweet "The plan is, we keep [the Workbench API] as open as possible. It has to be. It's something for you guys to use, so you guys need to have a say in it." He continued with, "It may not have much to it for a while because this is very (very) early stuff, but after the stuff with 1.3 calms down we're focusing on it. And as a lot of people keep asking how flexible it'll be; we eventually want to start using it to add features ourselves. That flexible. For now we're going to focus this on servery things until we rewrite the rendering engine of the client, then we'll make it client too."
Also, in reply to a user on Twitter, he revealed that mods for the Workbench API will need to be written in Java (no surprise there), but with the ability to write loaders for other languages (say, C++). If you don't understand any of this, it just means that mod developers will get to do even more amazing things once the Workbench API is implemented.
Source: @Dinnerbone