What became of data over mains cabling?

I remember hearing a few years back about a plan to use the mains power grid as a data network; apparently, using multiplexing or some such, the bandwidth of the system was going to be phenomenal, then nothing happened.

What became of the plan? How would the bandwidth compare against DSL?

Well, it is alive and well for in home Networking; re How Stuff Works
How Power-Line Networking Works

I used to work for a company that developed and manufactured Power Line Carrier (=“data over mains”) equipment.

Electric Companies were using PLC to temporarily shut off power to (suitably equipped) electric water heaters, sometimes air conditioners, and other “deferrable loads” during times of high demand.

I would be surprised if anyone could build a case that PLC could be useful for a data network. Even in the most favorable circumstances the best signalling rate achievable was under about 1000bytes/second - far slower than even a poor dial-up modem. And this required a 200W-1KW transmitter to reach any useful distance.

The trouble is that powerlines are “noisy”. Worse, the power company places devices called capacitors on the lines that improve efficiency of power distribution, but kill any data signalling.

Home networking is a little more feasible; shorter distances, and the local circuits on the same distributiion transdformer (those trash-can-looking things you see on utility poles) don’t have to deal with local signal-killing capacitors. I might worry about your neigbors snooping on your signals though.