Please note that MEncoder is not an actively maintained application (you should try FFmpeg for most purposes) and cannot use VDPAU (and should therefore not be discussed here).
I understand. Good to know. I did not know that and I am looking to use ffmpeg for encoding/transcoding applications in the future...
So I have been playing a video (bluray) with mplayer and experienced no errors or problms at speed 0.5...
The only message I saw was when I forwarded the playback with the mouse scroll:
[h264_vdpau @ 0xe044e0]number of reference frames exceeds max (probably corrupt input), discarding one
Except that, nothing else. the command I used was:
mplayer -vo vdpau -vc ffh264vdpau -ao alsa -speed 0.5 $movie