They don't work on it, or at least made it clear they wanted to avoid any misunderstanding on that matter: https://twitter.com/JollaHQ/status/388588752357953536

However, it suggests that something similar to Lumia's glance could be achieved. Even with high energy consumption, I'd buy it. As stated above, it would still be better energy-wise than keeping full display (by preventing locked screen). Plus with proximity-sensor-based activity, this LPSish screen would only work a few minutes/hours a day, and only when necessary when the phone cannot be touched every 2 sec to check status and clock (car, bike, running, meetings, night)

Even with associated battery drain when on, such a feature would definitely be an improvement in my opinion. And if battery's a problem, then just disable the feature or keep your proximity sensor covered by keeping the device upside down or in pocket.