More practically, something like
ffmpeg -re -i file.mp4 -vcodec copy -an -f rtp rtp://127.0.0.1:10000
(with file.mp4 containing H.264 video) will not work.
I have a very elegant patch to address such issue, but the margin of this blog is too small to contain it... Seriously, I'll post it as soon as I learn how to attach patches to a blog entry.