The installer needs to remove operating system files from the install target, but was unable to do so. The install cannot continue
I was installing Ubuntu on top of a previous eeebuntu install, smashing the / partition, while reusing the /home. After minimal googling, I found this bug report at Launchpad, with the same problem (and one year old).
As it turns out, the problem was not with the root partition, as I assumed from the error message, but with the home one. Apparently, Ubuntu didn't like the idea that my home partition was JFS (maybe it couldn't mount it, because jfs_utils are not loaded by default). The solution: install the OS ignoring (not using) the home partition, and mount it afterwards.
Shame on you, Ubuntu, this solution is lame!