At the Virtualization Deployment Summit Microsoft and Citrix made the most explicit statements about their partnership they have ever made. There's a truckload of announcements that were made at the Virtualization Deployment Summit but these are the most concrete facts that apply to this new Microsoft / Citrix pact:

• Citrix and Microsoft will be delivering a joint VDI proposal. Citrix will deliver the VDI Broker with Citrix XenDesktop Server 2.0 and Microsoft will deliver the management tools needed with System Center Virtual Machine Monitor. Both Citrix and Microsoft deliver a Hypervisor (XenServer and Hyper-V)
• As an addition to the previous statement, Citrix will be developing a component that will allow virtual machines to be (seamlessly) interchangeable between XenServer and Hyper-V. This interoperability between Microsoft and Citrix products goes as far as Microsoft System Center managing Citrix Presentation Server and XenDesktop Server.
• Presentation Server (Delaware) will be the preferred platform if plain Terminal Services can not suffice (no surprises there)

I expected Microsoft and Citrix to team up ever since Citrix acquired XenSource . This feeling was only amplified when VMware announced that they had acquired Thinstall and there still was no VDI answer from Microsoft. Well, Microsoft's answer is there now and it's a very persuasive answer. Will VMware be able to fight of this Microsoft-Citrix tag team? I don't know. The one thing that VMware has going for itself is that they have a full VDI solution themselves, whereas Citrix and Microsoft (especially Microsoft) needed to team up to be able to deliver a complete enough solution. And of course the VMware Hypervisor is still considered to be superiour to both XenServer and Hyper-V but it is going to be a real big battle for VMware to succesfully take on these two giants. 

So like I said, this all makes perfect sense. There's only two things "bothering" me about all this.

1. How will Microsoft position the technique they acquired from Calista Technologies? I would be kind of weird for them to steal Citrix' thunder in the VDI game, I mean ICA is one of Citrix' strong points in it's whole VDI propesition (not it's hypervisor for example).
2. Why did Citrix buy XenSource? If Citrix was already planning to team up against VMware with Microsoft (and I am assuming that they knew this before September '07) why would they want to buy XenSource, especially at that high a price? I can not find a good reason.

In a very tiny nutshell it comes down to that Microsoft is using XenDesktop Server 2.0 as the Broker of choice in their VDI proposal. They have their own hypervisor and with the acquistion of Calista Technologies they also could potentially have their own "ICA protocol". They already had the management with SCVMM. Usually Microsoft just buys that missing component but not this time. Perhaps the time wasn't right yet. I think it will be in the future.