I found this thread after I installed openshot and started playing with it, and preview wasn't working for me either. Since I'd also installed Debian squeeze on another partition to play with KDE I tested to see if it worked on squeeze, and discovered it does! It's a different version, however... I'd read a thread on here about firefox and pulseaudio I think, where someone noted that their CPU% went way up or maxed out and I noticed that one of my 3 cores went up to 100% in htop when I opened a project in OpenShot in lmde, but it didn't do that in squeeze.
So I realized that I had only installed alsa on squeeze but no PulseAudio; so to check out my theory I removed PulseAudio from LMDE 64bit, opened OpenShot and a small project and without pulse the CPU doesn't go up to 100%. And preview works! And since my project was just playing around with a few image files; I rendered it in youtube at low quality and it rendered quickly. That was the only time one of the cores went to 100%. So in short, it's some sort of issue of how it interfaces with PulseAudio, apparently. I looked and didn't see any option to tell it what audio to use (alsa vs pulse), like I did with VLC. It may be worth a bug report somewhere, I think I'll just leave pulseaudio off for now and see how that goes.