gst-av is a GStreamer plug-in to provide support for libav (formerly FFmpeg), it is similar to gst-ffmpeg, but without GStreamer politics, which means all libav plugins are supported, even if there are native GStreamer alternatives; VP8, MP3, Ogg, Vorbis, AAC, etc.
In addition, it is much simpler (2654 vs 16575 LOC), has better performance, and has a bit of extra features (such as less latency), and doesn’t use deprecated API’s. In a previous post I measured exactly how much improvement compared to gst-ffmpeg there is; it’s not much, but it’s some.
IOW; it’s possible that gst-av is the only GStreamer codec plug-in you would ever need 
Compared to the previous version, now there’s video decoding (H.263, H.264, MPEG-4, WMV and VP8), and video encoding (H.263 and H.264), plus a fix for audio decoding with playbin, and better integration with the official FLAC parser.
For Nokia N900 users, ogg-support is going to use gst-av for 1.0.7, which you can test now, now.
BTW. It works both for libav and FFmpeg.
There’s now a project page hosted in Google Code where you can find all the juicy stuff.
Enjoy 
As a person looking from outside the project it looks to me that (former ffmpeg) is the wrong term, as ffmpeg is still ffmpeg and libav is a fork of it. For me it feels like the ‘former ffmpeg’ will just add to the hostility between the projects instead of helping for some reconciliation.
@Mark Michael Niedermayer is the only one that wanted the fork to happen, there will be no reconciliation until Michael Niedermayer steps down as leader. I doesn’t matter how I (also an outsider) describe libav.
I tried the latest ogg-support and unfortunately despite my best effort it just breaks FLAC playback completely. Had to revert back to the previous version.
@Alex You mean 1.0.7~rc1 or 1.0.7?