EC2 only lets you export instances as VMWare-compatible OVA files if you originally imported that instance from an OVA. Presumably it preserves the metadata and XML gubbins for the instance, and just wraps it up again using that metadata on export.
In order to provision arbitrary VMs in an OVA-exportable way, we abuse the volume snapshots on one VM.
- Make a fresh install of ubuntu server or whatever your base distro is, in VMWare, export as OVA file. (single disk only!)
- Untar the OVA and import the VMDK file into ec2 using
ec2-instance-importonto an HVM instance type (ie, no xen kernel needed)