They won't have to make theirs open, or join OpenStack. All they would have to do is write a set of binaries that translate commands from one to another, and translate return values. Presumably some hackers will sort this out and open source it.
boto, libcloud, etc, etc mostly support talking to multiple clouds. The API is far from the hardest part of migration - it's actually pretty simple in most cases, unless that vendor picks something like SOAP.
