|The big problem with mowing is that you risk spraying the urushiol oil into the air. I agree, Brian, that a careful plan is in order.
How big is the yard? Could an afternoon be spent pulling up the poison ivy (carefully and with the proper setup)? I've done that when the plant just couldn't be eradicated from around my azalea bushes. In order to get rid of the plant, you've got to make sure that you get rid of the roots. Herbicides do take time to work as you've noticed.
Something else you might consider is mowing and then applying Brush-B-Gon. You've damaged the plant and then applying the herbicide could help kill the roots.