A couple of comments with which you are free to disagree, particularly the homeowner. Poplar is good painted since it really doesn't have a particularly interesting grain or pattern but I would not use it for a floor since it is relatively soft. I definately would not use generic pine from the lumberyard since that is really soft and doesn't sand all that well. With second growth farmed pine, the annual rings are real wide and have a marked variation in hardness that makes them tend to be wavey after sanding. Certain pines, like Southern Yellow Pine, make great floors but you want to do your homework on exactly which pine you want and which will work well.
Painting floors is OK as long as you like to paint. If you paint your floors, the chances are good that you will be painting them again a lot sooner than you expected. Two or three coats of polyurethane can save your floors from a lot of abuse and damage for a long time. Runners and area rugs can save your floors from a lot of damage and extend their life too.