The porting stories have come a long way, right from Android phones to Nokia and even the iPhone. It looks like people have also managed to port MeeGo and Ubuntu on the Nexus S. While MeeGo is a Nokia-Intel mobile OS, Ubuntu is a Linux Desktop OS.
According to Steven-Troughton Smith (the same guy who ported MeeGo to HTC HD2), the Nexus S can run MeeGo. You can check out how to do so on his blog, which involves building a MeeGo rootfs in ext2 format. The porting led to the screen going dim and although a modification to the kernel did the job, the screen now has a tint of yellow. Nevertheless, it’s working fine.
Apart from that, the X11 and UI apps as well as the ADB root shell are working. So while this much is okay, the Touchscreen and Wi-Fi don't work once ported. If you are trying to run any of these on your Nexus S, make sure you have the technical know-how to do it.
Steven also tried porting Ubuntu to the Nexus S through the same procedure and says that it works perfectly well. Check out this link for more information.