Rule to delete other node of same type before saving content
Daniel,

What I think is happening is that you're trying to delete the actual node that the user is about to save (it hasn't been saved yet)!

It doesn't look like you're finding the user's PREVIOUS node to delete first, you're just trying to work with the current one.  :-)

Of course - since this current node hasn't even been saved yet (your event is prior to saving), there is nothing to be deleted.

The rule will have to be more complicated, with your finding an existing node by the current user first... 

Hope this helps.