[Mickydtron]

It would be hard to detect the best time to deliver the message. Naive approaches could get it almost exactly wrong, as one of the times when I really don't want to be interrupted is when I'm just sitting and reading something, and not really interacting much. I'm sure there are better ways of determining if the user isn't focusing on anything, and maybe some of them would work well as a trigger for delivering messages. That would be pretty nifty.

[nxzero]

Correct, it would be hard, which is why if you we're able to figure it out, it'd be useful and hard to copy.