This is big news. You can still purchase a license for it, but instead of enabling features, it only enables you to use Citrix support and apply hotfixes with XenCenter. Hotfixes can still be applied without the licensing, but you have to use the command line to do it.
I've been running a server here with Ubuntu for the host and using the XAPI package to do Xen virtualization (and use XenCenter) but it never really worked right. You could see the virtual machines and control the mouse but keyboard events never made it through.
Hopefully I can find time this week/weekend to install XenServer 6.2 on it.