MIUI is not the ROM for everyone, we get that. But for those that do enjoy using it, things just got a little better -- Xiaomi has decided to release the source code changes and is in the process of open-sourcing the project. As of the time of this writing, you'll find the core patches to things like the framework and the make files as well as code for the file explorer, notes app, compass app, and sound recorder at MiCode's github pages. An official sounding announcement on the MIUI Android blog says to expect code for more applications in the "very near future". Hooray!
MIUI is more than just a custom ROM. Xiaomi builds and releases phones in China running the heavily customized version of Android, and through the hard work of their development team and independent developers, the builds have been modified to work with many phones we're familiar with here in the rest of the world. It's a big change from the stock look and feel of Android, and some say it has a very Apple-esque vibe about it. Have a look at a review of MIUI for the Nexus S and see what we're talking about. This is a far cry from a developer adding in a few custom tweaks to the AOSP -- we're talking a very big rewrite much like HTC, Motorola, Samsung and LG do with their custom builds of Android. The difference is that MIUI has now decided to share with everyone.
Building MIUI isn't going to be something that most of us will do. But your favorite ROM developers, especially those that work with MIUI for various devices already, now have a whole new toolbox at their disposal. Look for more, and better, builds of MIUI to come from this, as well as some features and options from MIUI to make their way into more customs ROMs everywhere. Good news all around.
Source: MIUI Android