An open-source project is more than the code. It also includes the community and the resources for continuing development. Although the code is under the GPL, Sun (and now possibly Oracle) has control of the brand and many of the potential revenue streams. A fork would lack the benefit of a clear, centralized community and many resources to fund development. Both of these things threaten the future of MySQL as an open-source project.
Forking + rebranding is perfectly viable option. A similar thing happened to the popular Mambo CMS, which had a big community and user base. Over time, everyone switched to Joomla, the rebranded free version - I don't see Mambo mentioned anywhere anymore.
If the same zeal and effort being spent to oppose Oracle were being spent to create a community around a fork, helped by the controversy it'd be a widely known thing already. Couple that with OS vendors choosing the fork over the "mainline", and in a few years the fork would be "the" MySQL, albeit with a different name.
