According to the recent research paper Fine grain Cross-VM Attacks on Xen and VMware are possible! published by WPI has proven a working attack that allows a malicious user to capture AES encryption keys (and more) from other VMs on the same physical server, even if they are using separate CPU cores. This vulnerability impacts VMWare and Xen and was tested out on Amazon and Rackspace two leading cloud providers. Both of which proved vulnerable.
The short of it is that encrypted credit card data (for instance) is not safe in most shared Cloud environments.