The cruise lines tend to upgrade people who have sailed with them before, or people who booked with a high volume travel agent, or people celebrating special occassions (honeymoon or anniversary), or just random people.
I would think that they would upgrade people if the rooms are empty, but you never know. I have seen that they try to sell the upgrades at the pursars office if there are empty cabins.
If you have seen the price of your cruise drop dramatically since you booked then call your travel agent and ask if he/she can get you an upgrade or a cabin credit. I did this yesterday and got a one catagory upgrade on my sailing in December.