If there was some way for somebody to make an SMP mod that allowed you to look at a block and type /ident or something and it told you a list of the last 5 actions on the block and who did the actions. that would be awesome.
You'd have to make software to keep track of actions regarding EVERY block in the world, which would just eat up memory. I don't think it's practical until Notch lets us have finite maps in SMP.
[Request] Block placement identifier