Agreed! The Android is released under the GPLv2, which clearly states that you must release the source and license of the original code and that if there are any modifications to that code then you must include the dates of the modifications. If there are some parts of the MIUI rom that has been made independent from the Android OS, then those parts who's source is not released under the GPL must be distributed separately, as per section 2 of the GPLv2.
I have looked all over and it I cannot find any of the source for this ROM. This is in clear violation of the license in witch Android is released.
I feel, personally, that if the MIUI developers are not releasing there source then there is now way for the rest of the community that uses it to know for certain that this software is not malice by nature. It would appear that there is a good number of individuals working on this project, judging by which the speed of improvements are being released. What motivation could a team of people be working under when none of them are interested documenting what they are doing to the public at large. This is, in my opinion, a very dangerous mix. Your phone contains the most sensitive data that you have. As such I would expect the developers of MIUI to include the rest of us in the development process not just for the outstanding improvement of having more people work on a project, but to allow the code of something that is at the center of our private information to be reviewed by the people using it.