flash (macromedia):

I have faked it with actionscript. I just made the flash movie play at the same
framerate as the FLV - then on the frame with the FLV I added a moivelcip which
is 2 frames long. The first frame stops the movie, the second frame make it
play. This movieclip loops, causing a stop/go effect on the flv - which makes
it play at 1/2 speed. I may have the flv and sample still:
http://www.johnkalnin.com/flashhelp/FLV-HalfSpeed.html
http://www.johnkalnin.com/flashhelp/FLV-HalfSpeed.fla

You could always load your swf into another one. You see, when you load a movie
from inside another movie and they have different framerates, the framerate of
the outermost movie is used. The sound could go out of sync, though.

If you look at my code I disabled sound when it goes to 50% speed because it would-sound-very-wei-rd-stop-ping-and-then-star-ting-and-then-stop-ping-and-then-star-ting.....]