This is probably a question that should be asked on serverfault.com, but let me try to answer that question regardless.
I use rackspacecloud, they're a cloud provider, although what they offer are very similar to what VPS providers offer, they just do billing by the hour and allow you to automatically provision new servers if needed.
You have the ability to backup a server and store it on their cloud files (the equivalent of Amazon S3) and spawn more servers based on that backup which is essentially an image of the server.
This can be used only on rackspacecloud, but since they're working on open sourcing their infrastructure to ideally disable vendor lock-in, we might live to a brighter future where people will be able to own their images and move them around regardless of the provider.
Additionally if you setup the VPS yourself (using a dedicated server and openVZ), you can backup the server and move it around to another server with openVZ installed, knowing this is possible (since openVZ is in fact abstracting the server hardware from the VPS), you should be in theory able to move that image to any provider who is based on openVZ, but good luck with that.
Hope that helped!