Markus Persson, the creator of Minecraft, has revealed on his personal blog that the game is set to support custom mods - although there are a few things that need to be taken into consideration.
Firstly, the mods will only be available to those who have bought Minecraft and you won't be able to sell or make money from your mods unless you've purchased a licence from Mojang, the parent company. They will also retain the rights to anything you make, and can use it themselves if they want.
You can find out more information about the proposed changes over at his blog - but it looks as though Minecraft is going to get even more creative.
Source: The Word of Notch