Tested libx264-98 2:0.98.1653+git88b90d9-3ubuntu2 and works for me on Maverick x86_64:
ffmpeg -i IronMan.mkv -an -vcodec libx264 -vpre medium -b 1000k -threads 0 proposed-b.h264 15M proposed-b.h264 = ~1132 kbit/s
Tested libx264-98 2:0.98. 1653+git88b90d9 -3ubuntu2 and works for me on Maverick x86_64:
ffmpeg -i IronMan.mkv -an -vcodec libx264 -vpre medium -b 1000k -threads 0 proposed-b.h264
15M proposed-b.h264 = ~1132 kbit/s