The developers at XDA have been working on getting Android to run in the HTC HD2 for quite sometime. In their quest, they have reached a major breakthrough by installing and running Ubuntu in the 4-inch device.
As you can see in the picture above, it is the desktop version of Ubuntu that is running.
Although they have reported that Ubuntu is running smoothly, Ubuntu is by no means a good operating system for a phone. Ubuntu is a Linux distro which has been designed for desktop and laptop computers not for phones. So even though it runs fine, you cannot do many functions crucial for a phone like making calls, texting etc. Moreover the interface has been designed for larger screen and will be a real pain to use in a 4-inch screen.
Nonetheless running a desktop OS on a phone is a really great achievement.