Good info on this in two places (rollout announcement and FAQ), but the bottom line is that only two flavors of smart plug integration today TP-Link HS110s and Wemo Insights, plus Hu e which is a different kind of integration. @RyanAtSense explains why these two smartplug models vs. the panoply of others available here:
“We know that these are not the only smart plugs on the market, but we chose to focus on them for a two main reasons: (1) they are both popular models in the market, and (2) they report not only on/off data but also wattage. If other smart plugs meet these criteria, we will consider adding support for them as well.”
From SmartPlug rollout announcement on the forum.
I think there is also a third criteria today as well - the smartplug must communicate via WiFi vs. Wave or Zigbee.