Q: Can I use Windows Azure as the host for an SMB share to use for my failover cluster witness?
A: Although technically the answer is yes, since a virtual machine (VM) can be created in Windows Azure that can host a SMB file share, and then that Windows Azure virtual network can be connected to your on premises using its site-to-site gateway functionality, in most cases it wouldn't be practical. Especially if, as is most likely, your desire to use Windows Azure is because you have two datacenters hosting nodes and want to use Windows Azure as the "third site."
The problem is, at time of writing, a Windows Azure virtual network supports only a single instance of the site-to-site gateway. This means it could only be connected to one of the datacenters.
If the datacenter failed, the virtual network connected to the other datacenter would have no access to Windows Azure. The virtual network thus wouldn't be able to see the file-share witness and therefore not use its vote and make quorum, rendering it fairly useless. Once Windows Azure eventually supports multiple site-to-site gateways, then using it for the file-share witness would become a more practical solution.