Eroding that boundary can also be in your favor... I can step away from the desk for a few minutes to start my laundry, prep something for dinner, or even just go to the grocery store during "lunch" because I can bring the food straight back home since I'm not far away from home at an office. Working remotely is giving me back time. And this isn't time "lost" from the employer's perspective... I'm just doing something useful with my break times rather than wandering down to the water-cooler to chat with other employees.
The trick is to allow only the erosion that you find acceptable. That's a matter of personal organization and self-control, and each person has to set up a system that works best for them. I use a spare bedroom as my office, and I only go in there during working hours. Everything work-related stays in there.