In my case I just built ffmpeg and x264 from svn and git repos, and configured apparently well.
this is my ffmpeg configuration:

configuration: --prefix=/usr --enable-shared --enable-libmp3lame --enable-gpl --enable-libfaad --enable-libvorbis --enable-pthreads --enable-libfaac --enable-libxvid --enable-postproc --enable-x11grab --enable-libgsm --enable-libx264 --enable-libtheora --enable-libdc1394 --enable-nonfree --disable-stripping --enable-avfilter --enable-libdirac --disable-decoder=libdirac --enable-libschroedinger --disable-encoder=libschroedinger --enable-avfilter-lavf --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-version3

encoding to h264 with ffmpeg works perfectly well
(e.g.  ffmpeg -i INFILE -acodec libfaac -ab 96k -vcodec libx264 -vpre hq -crf 22 -threads 0 OUTFILE.avi gives me a nice video file, with video and audio)

The problem seems to be in MLT, as I can't encode to h264 using melt, even using the suggestions given on mlt mailing list, for example, i tried the following, and the results are always error messages and a file with no video and only audio:

melt -verbose  -profile dv_pal_wide INFILE -consumer avformat:OUTFILE acodec=libfaac ab=128k ac=2 ar=44100 f=avi vpre=hq vcodec=libx264 crf=22 bt=256 $(awk '{printf("%s ", $0)}' /usr/share/ffmpeg/libx264-hq.ffpreset)
[libx264 @ 0x81da0d0]interlace + direct=temporal is not implemented
[libx264 @ 0x81da0d0]interlace + weightp is not implemented
[libx264 @ 0x81da0d0]using SAR=64/45
[libx264 @ 0x81da0d0]interlaced (1) > level limit (0)
[libx264 @ 0x81da0d0]using cpu capabilities: MMX2 Cache64
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
|1=-10| |2= -5| |3= -2| |4= -1| |5=  0| |6=  1| |7=  2| |8=  5| |9= 10|
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
+---------------------------------------------------------------------+
|               H = back 1 minute,  L = forward 1 minute              |
|                 h = previous frame,  l = next frame                 |
|           g = start of clip, j = next clip, k = previous clip       |
|                0 = restart, q = quit, space = play                  |
+---------------------------------------------------------------------+
[libx264 @ 0x81da0d0]bitrate tolerance too small, using .01
[libx264 @ 0x81da0d0]profile High, level 5.1
[producer avformat] audio: total_streams 1 max_stream 1 total_channels 2 max_channels 2
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
[consumer avformat] error with video encode
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