Just announced at Teched the CU2 update for Exchange 2013 will possibly deliver Windows Azure for witness server for site resilience scenarios.
Until now for an automated site failover a witness server in a third site (datacenter or other well connected network independent location) was required. For existing Azure customers it will be possible to host this witness role on Azure as well. This will be a VM (fileserver) running on IaaS on Windows Azure. It says possibly as testing and validation is still underway.
This scenario may not be ideal for every company as it will require extending internal Active Directory permissions into a public cloud.
For a high available witness server 2 VM’s in Azure are required.
More details will follow.