Piercing a key defense found in cloud environments such as Amazon’s EC2 service, scientists have can extract private cryptographic keys stored on a separate virtual machine.
http://arstechnica.com/security/2012/11/crypto-keys-stolen-from-virtual-machine/
http://emea.emc.com/emc-plus/rsa-labs/staff-associates/cross-vm-side-channels.htm
http://dl.acm.org/citation.cfm?id=2382230