mplayer cannot play RTMP streams yet as it seems. I made some dumps with wireshark, and with tools, but mplayer wasn't able to play them.
At least the wireshark dump was recognized as video, so mplayer opened a video window, but the content appeared to be encrypted as it was just flashing colored blocks.

Hopefully, not all RTMP are encrypted, though. But I dread the day YouTube will switch to RTMP (and I'm pretty sure they'll do eventually with pressure from the content industry).