While this technology has been put to use in Europe, it has always been considered impractical in North America because North American electric systems step down to household voltage at the pole, necessitating a local transformer through which the network signal is destroyed. In Europe, the stepdown to household voltage is far enough away that it is practical to locate the off-grid network uplink after the transformer.
The Sault Ste. Marie solution is exactly the opposite: take it off the grid immediately before the transformer with a wireless access point. Data between the WAP and substations travel over the power lines, and a fibre link at the substation connects that end of the connection to traditional networking equipment.
The article suggests that this is advantageous because power companies have a last-mile advantage over phone and cable in some areas, but my inclination is exactly the opposite: the "last mile" here is the 150m range of the WAP. Phone and cable companies already have pairs going into houses that they can share for services, so there's really no last-mile infrastructure costs to provisioning DSL or cable internet.
But a 150m range means that the power company is going to need a WAP every 250m or so, and that sort of equipment outlay sounds a lot like laying a brand-new last mile to me. And Sault Ste. Marie is a small city: 74,000 people in 2001 with a 6000-person drop between 1996 and then. Pulling a 10m average frontage out of thin air, that means one WAP can probably serve about 50 dwellings (with overlap to the next WAP), which means you're looking at a couple thousand WAPs to serve the city, and that the MTBF of the wifi last-mile is 1/1000 or less of the MTBF of a single WAP. It appears you can find outdoor WAPs with 20kh-30kh MTBF, which means you'd be rolling a truck to replace a WAP every two days. Somehow that doesn't seem to scale to populations over one million.
On top of that, you've got a related business problem: neighbourhoods with low adoption rates cost significantly more to network. With DSL, the hardware deployment centres around the CO, and you can buy sufficient DSLAMs to meet projected need, wherever that need might come from -- but one customer in a neighbourhood means that that neighbourhood needs a WAP.
It's certainly a neat way of solving the problem of grid internet with transformers on the pole, but I can't see where it makes much business sense -- but I admit that it would be pretty neat to have your laptop just work anywhere in town that has power and a nearby WAP.