The other day Alex got a really good picture of what happens with laying workers:
The image shows the craziness of what goes on when a colony goes queenless for too long. Some of the workers start dropping unfertilized eggs, but they often plant several eggs in each cell. I've seen over a dozen eggs in some cells, but there are usually fewer than that. Of course the bees will abort multiple larvae in single cells--there just isn't enough space. Still, it's interesting to see three or four larvae for the short time that they are all curled into the based of a cell.
It is important, however, not to assume that anytime you see multiple eggs in a cell that it is laying workers. Young queens sometimes deposit more than one egg in a cell, and in the springtime the queen sometimes does the same thing on the fringe of the broodnest--just trying to get out more eggs than the bees can take care of at the moment. Laying workers almost never appear until all of the missing queen's worker brood has emerged.
I should really take a picture of the lumpy frames that develop when drone larvae pupate in worker cells that are too small for them. It's a mess, but not permanent--those cells will be fine for workers once there is a fertile queen.
There are two safe ways of requeening laying workers that I use. If there are a lot of bees, you can give them a queen cell and it is accepted. It is also possible to give them a new queen with a couple of frames of her own brood and bees. But trying to introduce a new mated queen in a cage is pretty risky--they usually will not accept her on her own.