I seem to remember something similar happening back in the 2.x days. Hope this is a one off, not a new 3.1.2 bug... I put a playlist on, all was fine, first track started playing for about 2 seconds, the ipod app crashed and when it came back up it was completely empty other than some podcasts I had downloaded direct to the device last night. In settings > about the disk space used was the same as before, the files were still there, but not showing in the ipod app. A sync with itunes seemed to rebuild the library, all the tracks came back without having to re-copy them all back to the iphone. This is at least better than the bug in 2.x which meant you had to re-load all your songs, which can take hours. Anyone else had this issue? One of my twitter friends had the same thing happen today.