Dogs will sometimes do this as a form of protest for being left alone, especially if they are high strung. Try taking your dog to the vet and see if he will prescribe something like valium to calm her anxiety. Behavioral drugs like this sometimes work in these cases.
According to Cesar Millan, host of The Dog Whisperer, dogs may defecate on objects in your house if you have not set proper boundaries and limitations for them.
Perhaps your dog is testing it's limits. Some dogs will defecate in areas that there have not been proper bounries set for them. They tend to try and "claim" the house as their own. You must establish yourself as the master of your domain or your dog will.
Perhaps you are not taking him outside as often as he requires to go to the bathrrom so he poops on your bed for attention! That would certainly get my attention :)
They think its their house and they have the right to do anything, anywhere! Let them know (kindly) who is the boss and who is the subordinate here. Maybe restricted access to upstairs for a while.
