Seth Forshee, a kernel engineer at Canonical since last year, published the Apple GMUX driver to the kernel mailing list. From the commit message, "Apple laptops with hybrid graphics have a device named gmux that is used for switching between GPUs and backlight control. On many models this is the only reliable method for controlling the backlight. This series adds initial support for the gmux device, along with anciallary support for disabling apple_bl when the gmux device is detected. Initially only backlight control is supported."
This new driver was originally posted last month, but since then it's gone through three revisions. The version posted on Thursday was the third revision that incorporated various changes requested by other kernel developers.
The driver amounts to a little less than 300 lines of code, but nevertheless it's a welcome contribution. Hopefully the Apple GMUX driver will be able to make it into the Linux 3.4 kernel.
Aside from Apple laptops, there is Bumblebee for providing some NVIDIA Optimus support while we're still not yet to a decent point with GPU hot-plugging support under X and other features, but slowly moving towards that point with mainling GPU hot-plugging in X, DMA-BUF, PRIME, etc.