Be it with Firefox 7, Thunderbird 6, 7 or on the trunk, the telemetry is never sent. I only got it sent once with Firefox 7 some days ago, but never since then. Since there's a 60 seconds idle timeout, I tried letting the computer idle for 5 minutes or even half an hour with no luck. The screensaver gets activated, but nothing is sent. I tried to register an observer for nsIIdleService, it works as expected, so that's not the problem. By the way, I wonder if the data currently sent by telemetry is very representative because the data from heavy use sessions won't be sent (since the user is never idle enough). Maybe a shorter idle timeout would useful. I also tried with a clean profile with just the add-on about:telemetry, still no luck.
We're pretty sure this has been fixed via changes to the idle service or similar. Please reopen if you are still seeing this.