Xen in mainline
Getting the dom0 stuff fully merged is a good thing, but if you recall the domU merge in ~2.6.24, it took until .27 or .28 for it to become usable, and arguably until .32 for it to become stable. I know that Linode has been testing this code for a long while now, but it has not yet survived to production.
With many more eyes looking at the code, I would expect the bugs to be worked out and enough testing will be performed to deem it production-ready within a year or so. At that point, Linode can slowly start introducing it into production.
(By "slowly", I mean "over a span of years"… except in Fremont, where upgrades can simply be applied the next time the power goes out.)