Broadwell support has been a work-in-progress on Linux for many months and most of the hardware enablement is complete. The Mesa driver has had mainline support for Intel Broadwell graphics for some time now, but only today is it being enabled by default and not hidden behind the Intel preliminary hardware support flag. The latest Broadwell work was with this commit and other changes.
There's already been numerous articles on Phoronix about Broadwell for Linux and with the Mesa 10.2 release it will be in great shape. In recent days there's been features to land like better performance with HiZ and Broadwell MSAA anti-aliasing, etc. These features are for Mesa 10.2 that will be released in about three months and not the Mesa 10.1 release due out at the end of February.
On the kernel side the Linux 3.14 kernel is in good initial shape for Broadwell hardware. The kernel support will continue to be improved and refined upon in the next few release cycles with Linux 3.15 already having some useful Broadwell improvements queued up.
From the sounds of it, Intel Broadwell CPUs won't be available until Q4'2014 after some delays so there's still more time for open-source driver support refinements to be made and for landing them into stable software releases.