Now that would be a good choice...
"If XenServer suddenly takes off, don't be surprised if Microsoft just whips out some cash and buys Citrix, despite all of its work on Hyper-V"
Seeing as Hyper-V is based on good old VirtualPC, which Microsoft bought then made over into something that is pretty much unusable (as I know only too well from painful and time-consuming admin support experience)... oh, well. I am currently working as a consultant for a company where Hyper-V is costing valuable man-hours every day because it just won't behave like a nice hypervisor; they're in the process of switching to Xen. My best guess is that if MS takes over Citrix, that will kill Xen dead in the market, with customers jumping over to VMware by the thousands.
Or to Zen, which would most likely experience a source split from Xen and remain a viable OpenSource alternative. *shrug* Any way you cut it, VMware has the most credibility behind it with Xen just catching up, so I'm hoping Citrix will not get eaten by MS and continue to offer what already is a good and stable (now there's a hint for the guys at MS...) virtualization package.