Amazon recently announced a new feature: importing VMDK images into EC2. Right now the support is limited only Windows Server 2008 SP2 (now that is specific!), support for Linux and other versions of Windows is coming soon.
There are some other limitations that can make a perfectly good VMware image unusable in EC2. Remote access needs to be enabled for obvious reasons. Also anti-virus and intrusion detection software cannot run during conversion process, but you can enable them again later.