I have the same issue. Screen saver seemed to pop up 'randomly'. Here's what I've determined:
MacBook Pro, 15-inch, mid-2010
2.4 GHz Intel Core i5
4GB 1067 MHz DDR3
Intel HD Graphics 288MB
Mac OS X Lion 10.7.2
Air Display 1.5.0
I had the screensaver to kick in after 5 minutes of inactivity. After looking at the Avatron website (makers of AirDisplay), I started timing the screensaver to see how long it would take to kick in. Turns out it kicks in after 5 minutes, regardless of activity or inactivity! Getting out of screensaver resets the clock. So it's not 'random' at all.
Also from the Avatron website: ( http://avatron.com/forums/read.php?9,14490 )
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We're still trying to piece together reports about this. Here's what we think we know, but haven't posted yet because we're not quite solid on the details:
It appears that with the early 2011 15" and 17" MacBook Pro models (the only Macs with the NVIDIA GeForce 330M), the screen saver (or screen lock) will kick in at whatever time it's scheduled for, EVEN IF you've been moving the mouse or using the keyboard. But as far as we can tell it's only on those models. The workaround (not necessarily recommended, depending on your usage patterns) is to set the screen saver duration to be longer than the sleep duration. So for example, sleep after 30 minutes, but turn on the screen saver after 1 hour. I think. If you could try this workaround, it would be great to hear your results.
Fortunately, we do have a couple of machines that we can reproduce the problem with, so we'll definitely try to isolate the problem and report the details, but we need to understand it a little better before we can do that.
It doesn't appear likely that this is something we'll be able to fix, as it affects machines with ANY third-party video driver installed, not just ours. I believe the fix will have to come from Apple, along with the fixes for a few other NVIDIA GeForce driver bugs.
More info soon...
Dave Howell
Avatron Software, Inc.
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Hope this helps other folks diagnose their problem.