Absolutely, the full implementation will allow each user to sign with their own keys, and provide user-configurable trust rules ("allow images only signed by this key"). The Docker CA will be used as a default convenience to provide a common namespace if you want it, but users who want to use their own custom PKI will have all the flags to do that, and there will be an "escape hatch" to opt out of the entire trust infrastructure altogether.

The only reason we're starting with verification-only, and only for images produced by the official library maintainers, is because the other side of the tools (signing) are not yet ready to be merged in Docker. By releasing a subset now, we can start getting some feedback and ironing out the quirks, while the contributors finish their work on the signing tools, using the library maintainers as guinea pigs. Hope this helps.

PS. to state the obvious, all of this is taking place in the open on #docker-dev in Freenode. It is being designed by key contributors from multiple companies, and you are welcome to join the fun.