ffmpeg recording webcam to h264 mkv

$ ffmpeg -f v4l2 -s 640x480 -i /dev/video0 -vcodec libx264 out.mkv
ffmpeg version N-40640-g5edd4fc Copyright (c) 2000-2012 the FFmpeg developers
  built on May 14 2012 02:03:22 with gcc 4.4.4 20100726 (Red Hat 4.4.4-13)
  configuration: --enable-gpl --enable-libx264
  libavutil      51. 50.100 / 51. 50.100
  libavcodec     54. 21.101 / 54. 21.101
  libavformat    54.  4.100 / 54.  4.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 72.105 /  2. 72.105
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 11.100 /  0. 11.100
  libpostproc    52.  0.100 / 52.  0.100
^CEstimating duration from bitrate, this may be inaccurate
Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 2785251.674570, bitrate: N/A
    Stream #0:0: Video: mjpeg, yuvj422p, 640x480 [SAR 72:72 DAR 4:3], -2147483 kb/s, 1000k tbr, 1000k tbn, 1000k tbc

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nothing happening until i hit Control+C 2 times, then lists some junk,
ffmpeg -f v4l2 -s 640x480 -r 30 -i /dev/video0 -vcodec libx264 out.mkv
records video if i specify fps: "-r 30"