The OpenStack community has recently released the Essex release, which supports XCP and XenServer. A number of vendors have worked at that support including Citrix, Internap and Rackspace Public Cloud. You can find some more information about Xen support in the OpenStack Essex release at this wiki page. If you are interested in what has changed in Xen support fro Essex check out this blog post.
Note that there is a roadmap session on XenAPI support in at the OpenStack Design Summit later today: XenAPI (XenServer/XCP) Folsom Roadmap at 2pm, PST. If you are at the Design Summit and care about Xen support, why not drop in and meet Ewan Mellor, John Garbutt or Renuka Apte.
Of course, project Kronos is almost completed, which will help Xen support in OpenStack. You can find information about XCP-XAPI in Debian in the Debian package repository (for docs see Debian XCP wiki and this tutorial). XCP-XAPI support in Ubuntu is near complete: we are waiting for the XCP-XAPI packages to be synced to Ubuntu (see ticket #962184). Documentation can be found in the manpages and the XCP-XAPI package description.