I'm on a PC and have an Android tablet and phone. At first playback worked on all devices (I did have to install a Java update on my PC and some Windows Media Player Firefox plug-in... but once I did that, I was good to go).
Then, after unplugging and repositioning the camera, the playback on my phone and tablet stopped working. Even old videos that previously worked, no longer played back. I'd get the following error message: "The current video clip is unable to play."
I tried a number of different recording setting, but nothing seemed to work. I then reformatted my SD card, and BOOM... playback resumed! No idea why, but it's working again for me.
Hope this helps someone. I'm recording with Video Profile 2 and the following settings: MPEG4, 320x240, 10FPS, CBR, 256Kbps, play2.sdp