Oh my goodness! How would you like your Nexus S, which recently of course booted up MeeGo, to boot you up some lovely Ubuntu? It's totally possible, and it's done by the same dude as did the MeeGo boot! XDA developer forum member, stroughtonsmith — or Steven Troughton-Smith brings us all we need to get started.
Mister Smith does note that even though you can do this on the Nexus S, it might be better to do it on your "personal, every day device." - meaning of course that you probably shouldn't do it on your mother's phone because she will totally flip out if you brick it all up.
The same Ubuntu image is used that was used way back when we did the HD2, and the same process as was used for MeeGo is used to bring this system to this phone. Be careful, and tell us what happens if your phone explodes! Check out the full instructions in the thread at XDA.