That being said, he does seem to be using it with intent! Today one conversation went thusly:
"Hi! Are you really saying hello?"
We've been playing tracing games on his magna-doodle of late, including tracing his hand; but we hadn't done it today. Today we had traced several toys when I said, without gesturing or leading him in any way, "Can I trace your hand?" and then he put his hand down! Next I said "Can I trace your foot?" (which, I might add, we had never done before) and he promptly put his foot down to be traced! Then, of course, it was nothing but foot-tracing for the next ten minutes....
So it's confirmed - we have not only language comprehension, but a couple of rudimentary words and the ability to have conversations.
Guess that's means no more discussing degree secrets in front of him ;)