There are phase bridges to ensure powerline signals are on both phases in the typical US house, and phase filters available that filter the powerline frequencies - they were originally conceived not so much for leaking out, but preventing noise from leaking in and interfering with powerline stuff.

X10 users have used them for years - you can find them with vendors that specialize in dealing with the X10 community or home automation; although with the wireless mesh networks like Zwave or Zigbee a lot of the powerline stuff has (thankfully) fallen by the wayside.

Another way to get wired internet without possibly running new cable is with MOCA - ethernet over coax. You can find cheap DirecTV branded MOCA adapters all over the place. Most are 100Mbps but if you watch the newer ones are gigabit capable.