Citrix XenServer is now free. They were never winning over VMWare users on quality, so now they're shifting focus to price. That may not be a bad idea in this economy, but my suspicion is that Xen was more worried about losing customers on both ends of the virtual spectrum.
One one side, there's the VMWare users who seem to be happy with what they have. On the other side, are all of the users of the open source Xen offering. These are the guys that are busy building front-end tools and APIs to improve Xen functionality. All of that development from different sources could lead to more competition with the XenServer product. If XenServer is adopted by more users, the demand for external development could dwindle.
Or, maybe they're concerned about Red Hat, since they announced their intention to use KVM as a virtualization solution. RedHat is the one Linux vendor that the server industry pays attention to. Their actions are closely watched, and then usually adopted by other Linux distributions down the line. You can still run XenServer, but new adopters and implementers will probably be enticed to run KVM since it's bundled with the OS.
Whatever customer base they're trying to win over, it'll be interesting to see what the adoption rates of XenServer are for 2009.