this link here: http://www.ubuntu.com/certification/make/Lenovo states "Pre-installed only" for the X220 leading me to believe I can buy this model with Ubuntu preinstalled. I've been unable to find such an option at Lenovo's website.
I looked around and there isn't a lot of info on that. It does appear that Canonical's collaboration with Lenovo is very recent, with the press announcement dated May 9, 2011, so hopefully Lenovo systems with Ubuntu will start appearing soon.
That said, I did find one vendor that will preinstall Ubuntu on the X220 HERE.
This may not help you but I say it anyway.
I'm sitting in front of an X220i student model (this is german but it doesn't matter for the components) right now.
I live in Germany and Lenovo has a special student programme here. So they sell Thinkpads with special configurations and really low prices and without operation system.
I installed Ubuntu 11.04 and everything just worked out of the box. So I can really recommend this notebook.
I don't know where you live but if you have a friend who is student in Germany, you may want to visit him now ;) ... or the other way round. But pay attention on not making it look like smuggling :D