For the past two and a half weeks there has been a discussion on the fedora-virt mailing list for making Fedora's kernel capable of serving Xen guests again. The Xen Dom0 support is in the Linux 2.6.37 kernel, but there are some Xen drivers missing that will likely not be merged until the Linux 2.6.38 kernel, which may push back this goal to Fedora 16. There is some discussion, however, on possibly back-porting some of these newer Xen patches into Fedora's 2.6.37 kernel.
Besides the kernel side of things, there's also work to be done in ensuring Fedora's virtualization utilities (libvirt, virt-manager, etc) are still in good shape for Xen and that there's an easy way to enable the Xen kernel support from GRUB without manually editing the boot-loader's configuration file.
Red Hat will obviously continue to focus on KVM (the Kernel-based Virtual Machine) as their primary method of supporting Linux virtualization with it being the default with Red Hat Enterprise Linux 6.0 (RHEL6) and having played a large role in Fedora for many releases now. This will not change. There's just some Fedora users with hardware not capable of supporting KVM (older CPUs lacking Intel VT / AMD-V support) or having other reasons for using Xen over KVM and the other Linux virtualization solutions.
Besides the aforementioned mailing list thread, there's also this Fedora Wiki page regarding Xen Dom0 support. Fedora 15 is expected to be released next May.