Amazon EC2 only provided VM Import at the time you posted your question, but the initial support for this long-awaited feature was finally released on May 25, 2012, see the introductory post VM Export Service for Amazon EC2 :
Service AWS VM Import enables you to import virtual machines in different formats in Amazon EC2, which lets you easily transferred to your local infrastructure virtualization Cloud AWS. Today we are adding the following element to this service. Now you have the opportunity to export previously imported EC2 instances back to your local environment . [emphasis mine]
Please note that import / export of virtual machines is still limited only to Microsoft Windows , in particular:
Windows Server 2003 (R2) Windows Server 2008 EC2 VMware ESX- VMDK, Microsoft Hyper-V VHD Citrix Xen VHD. , .
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