Minecraft 1.4.2, Minecraft 1.4.5, Minecraft 1.4.6, Minecraft 1.4.7, Snapshot 13w06a, Snapshot 13w07a, Snapshot 13w09a, Snapshot 13w09b, Snapshot 13w09c, Snapshot 13w10a, Snapshot 13w10b, Minecraft 1.5, Minecraft 1.7.9
When using the command block to set your spawn or sleeping in a bed, when you die, you will spawn at the original spawn, and in the chat it will say: "Your bed was missing or obstructed".
11.11.12: I tried the same on some of my other worlds, and I had no problems.
The map was created before 1.4, in 1.3. Also, when I tried it on the surface (the problem is in a cave), it worked out fine...
11.15.12: Updated to 1.4.4 today, still not working...
11.22.12 Tried in 1.4.5 today, had the same problem... I wonder if it has to do with the location of it, i'll try moving my dungeon (thats where the commandblock is) further down or away from the previous location.