Using libx264 codec with ffmpeg

This is the information about the input file :

$ ./ffmpeg.exe -i input.avi

Duration: 01:47:20.81, start: 0.000000, bitrate: 909 kb/s
Stream #0.0: Video: mpeg4, yuv420p, 560x304 [PAR 1:1 DAR 35:19], 23.98 tb(r)

Stream #0.1: Audio: mp3, 48000 Hz, stereo, 32 kb/s

then
$./ffmpeg.exe -i input.avi -ss 0 -t 20 -vcodec libx264 out.mpg

then I tried to play out.mpg with VLC, but it only played the audio only.
Winamp, Windows Media player couldn't even play it.
Uploading to Youtube : Failed (invalid file format)
Used as a source file to a videoDisplay object in Adobe Flex,
: unable to play the file.
Uploading to Vimeo : There was a problem converting this video. Please contact vimeo help !