It just becomes detatched from your subdiv model. If you open up the hypergraph will will see its node, grouped with the subdiv. It can be safely deleted. Also watch out when going back into poly mode, as usually a load of poly blind data nodes are created, I usulay delete these. The perfomance hit you can take working with subdivs means taking good care of history and associated nodes, as it helps a lot. As far as I'm aware it has no further bearing on the models poly count, but it is a pain if you go back into poly mode as you will have two poly meshes around the model.
Hope that helps.