A moderator on unoffical Miui India forum has found a way to unlock the bootloader without need of any code or confirmation from Xiaomi. This method DOESNT involve Mi Unlocking tool. Many users have tried it and unlocked their bootloader successfully and quickly.
Edit - If you are an Indian user with the Snapdragon variant and if you have got permission from xiaomi and want to unlock by official method then you will need to flash China developer rom by mi flash. You will able to unlock bootloader on China dev rom by official method. Some users have also been able to unlock officially on Global developer rom. So take your pic. But don't waste your time on Global stable... Official method of bootloader unlocking will NEVER work on Global stable.
(I will update the OP if by miracle people are able to unlock officially on Global stable. Chances are very slim though.)
The method given below is for other global or beta rom users for whom normal unlocking is not working or those who are too impatient to wait for confirmation sms from Xiaomi.
Edit 2 - I HIGHLY RECOMMEND you to try this tutorial with GLOBAL STABLE FASTBOOT rom. People who have tried this on China dev or China stable have faced bricked devices.
Here is the forum link where bootloader unlock tutorial by hacking method is given.
Edit 3 - There is a difference between unlocking the bootloader by this hack and unlocking by official method. A big difference. I suggest you to read threads here and on miui forum as well to acquaint yourself with it.
Basically if you unlock bootloader with official method your system accepts it and causes no problems while flashing miui roms.
But if you unlock bootloader by this hack method, your system doesnt accept it fully and causes problems if you flash stock miui. Flashing miui without proper modifications will easily cause hard brick. My explanation was very simplistic but there are a lot of complicated things involved which go on at the system level.
If you decide to unlock bootloader by the hack method.. Read this thread on how to flash miui roms.
Also you don't need to worry about CM and AOSP roms. Whichever method you use to unlock your bootloader, CM and AOSP will work good without causing brick. You only need to worry about miui roms if you have used hack method.
My personal advice to you is that you don't try this hack method unless you are fairly well acquainted with technology and won't go mad if your device gets bricked. A lot of people have faced bricked device and many of them are noobs and I feel sorry for them. So unless you are courageous enough and feel confident, don't try this method.
Wait till your device can be unlocked by official method. That is best for new people.
Edit - This thread is outdated now. It was made a long time ago when development was in its infancy. There are lots of good threads on xda that can help you out. Ignore this thread and don't consider it's information relevant anymore.