Today at Teched in Barcelona Microsoft briefly mentioned cloud witness during a Storage replica demo in the keynote. I was wondering if this was somehow limited to this specific scenario. So I checked this new feature with Mark Russinovich who is now Chief Technical Officer of Microsoft Azure. He confirmed that the cloud witness feature will work with any fail-over cluster scenario, so also for Microsoft Exchange, meaning in the future you can have a stretched Exchange cluster with nodes running in 2 separate datacenters (2 sites) supplying quorum by a witness in Azure (3rd site). This provides much easier implementation for site fail-over and recovery.
The option for a witness in the cloud has been a much requested feature for years now, so this quick mention in the keynote is a huge announcement in my opinion. 