There are many registered trademarks in the open source world, including Linux, Ubuntu, and Wikipedia among others. OpenStack is also a registered trademark. I don't see any irony at all. Registering a trademark is a way to protect an open source project from abuse by people who would take advantage of the name to endorse their own unrelated product. Try getting the source code for aws and try getting the source code for open stack, and then tell me which one you think is more open.
The trademark will transfer to the foundation. It's just a way to ensure that anyone claiming to run an OpenStack compatible cloud is indeed doing just that. It's good for an OSS community to trademark brand assets and protect the name and meaning of their project.
Oh the i®ony.