I'm using sunabs svn PPA on Natty 64bit & I can't get x264 encoding, there's a big red cross against them in the render options. :-)
I have libx264-106 & 115, x264 Ubuntu package 2:0.106.1741-3, libmp3lame0 22.214.171.124ubuntu1 installed.
I have ffmpeg 4:0.6.2-1ubuntu1, libavcodec-extra-52 and lib avformat-extra-52 installed.
Using sunabs 32bit svn build on my laptop does give the x264 render options, so I guess I have some kind of mixed up dependancies on the 64bit machine.
I also have ffmpeg from git compiled from source with x264 support at /usr/local but assume kdenlive is not using that.