Yes, you could do this with something on Craig's List too.  Some of it would depend upon the triggering mechanism and how light-weight you would need the software stack, and to make sure you aren't tipping off somebody looking for messages of that nature that just seem a little bit odd.  A coin transfer would be harder to spot as something a little bit off and I think you might be able to create a lightweight version of Bitcoins that could be smaller than an http scanner to act as a trigger.

Anyway, for those who are into terrorist activities, I'm sure you can come up with dozens of ways to accomplish that kind of mechanism.  Perhaps some day I'll set up something that will release my blueprints to the Area 51 facility that will be untraceable.