AMD Hainan is rumored to be the performance GPUs making up the Radeon HD 8800 series with the HD 8850 "Hainan Pro" and HD 8870 "Hainan XT" products initially.
A few months back AMD provided initial AMD Radeon HD 8000 series Linux support for their open-source driver. Being committed across the various repositories today is now the Hainan support.
The open-source AMD Hainan enablement isn't anything huge and exciting but is just building upon earlier work. The commits today are basically introducing the family name and product PCI IDs. As shown with the DDX driver, the initial Hainan PCI IDs are 0x6660, 0x6663, 0x6664, 0x6665, 0x6667, and 0x666F.
When these graphics cards appear, they will now work on the open-source driver stack -- assuming you have the very latest Linux kernel, libdrm, xf86-video-ati, and Mesa driver components. There will also be same-day AMD Catalyst driver support as usual. For high-performance GPUs, however, I sadly still have to stick to only recommending the closed-source driver be used due to the large performance disparity and other missing features that make using a several hundred dollar graphics card on a feature-limited driver nonsensical.