The support for Windows XP is ending on April 8, 2014. The new Ubuntu LTS is coming on 17 April 2014. It's only 9 days of difference. I think that XP users genuinely interested to switch over the Ubuntu family can wait for this short amount of time after the support of XP is ended.
At any rate, users of PC with 256MB should consider using Linux Puppy or DSM (Damn Small Linux) instead as I think that even Lubuntu is more apt to perform decently on a 512MB RAM system at least. PCs below 512MB are becoming scarce in use these days anyway.
It is my understanding that most XP users today have computers ranging between 512MB and 2GB of RAM. Which is what Lubuntu and Xubuntu are best at. Above 2GB you can run Windows 7 comfortably and so that's probably where these users will be going to.