Intel has been working on G45 VA-API acceleration support for quite a while and had already delivered Arrandale / Clarkdale VA-API H.264 support and even more recently Video Acceleration API support for the latest Intel Sandy Bridge hardware, but they've now gone back to finally deliver on their G45 Linux support promise.
In April we then reported that G45 VA-API support should arrive in the second quarter. With weeks left to this quarter, it looks like they'll make it.
Created this week in the public libva repository for the VA-API library is a g45-h264 branch. This code branch contains this code commit that touches over 1,000 lines of code in the i965 VA-API driver for supporting H.264 decoding on Intel G4x series hardware. This is currently limited to a single thread, but it's publicly available for those interested. It doesn't appear that any Intel DRM driver update is required on the kernel side to take advantage of this video playback acceleration on the older hardware.
This work should allow more of the H.264 video playback process to be offloaded to the Intel 4-Series IGP rather than the CPU directly for multimedia applications that support the VA-API interface.
The Intel Sandy Bridge hardware not only has the full realm of VA-API support for video decoding, but it also supports VA-API video encoding too. Don't expect VA-API encoder support, however, to come to older generations of Intel hardware.