Using the latest SVN Kdenlive and git MLT (as of the time of this writing), attempting to render anything using libx264 produces a tiny file containing audio only. My x264 --version is "0.75.1251M e553a4c" and ffmpeg is built with --enable-libx264. libx264 shows up in the supported codecs in Kdenlive as well.
Using the "File rendering" destination I can render MPEG-2, MPEG-4, XVid4, Flash and Theora with success. The RealVideo files also have no video, but I'm not concerned about that (I'm on amd64 and Real stuff usually doesn't work anyway).
Please let me know what else I can try or what info I can provide to help. Thanks.