I've discussed the stewardship of the mobile with some of the community leaders in Canonical who are looking to help maintain the tag in Ask Ubuntu. There is nothing special about this tag, it's actually just like any other tag ubuntu-one, kernel, application-development, etc. The only difference is a blog post announced that this is where to ask questions about the now available Ubuntu Desktop for Nexus 7 device.
What I mean by nothing special the tag follows the same rules as all others on the site. Bug reports are off-topic, speculative questions about features that may be coming are off-topic, and all other policies we've maintained for the past two years that have made our site focused and high quality still maintain.
The Nexus-7 and Canonical "Mobile Team" have also committed to maintaining the tag (like Ubuntu-One, Kernel, and Application Development). Monitoring it for new questions and providing support and answers while also flagging and helping remove poor questions.
I also hope the community will apply the same tender loving care to incoming mobile questions as they do others. Editing them to make them more appealing to the site's policies and the community at large when applicable.
While Canonical/Ubuntu have done nothing wrong in announcing this, as the tag already existed and they're just taking it over in the form of applying man power to monitor the tag and driving more traffic to the site which will only provide us some high quality answers and questions, I've asked they consider providing a meta post even in the form of a quick PSA about the intentions. Ask Ubuntu is new to the Ubuntu ecosystem and every day is a learning opportunity or us and them.