It actually took a while for me to clue into what happened. Then, of course, I tried to get him to do it again to no avail. That is, until DH and I were getting him into his PJs, and he did it again. I asked DH "did you see that?". He said he did and it was quite obvious that's what Evan said AND meant, since he looked right at me, waved his hand, and said "hi".
Now that I've had more time to digest it, I'm realizing what an amazing moment that was. That was the first time in Evan's life that he communicated with us using English language in context (he's been saying Dada and Mama for a month or so, but most of the time it's not to either of us, so that doesn't really count). I'm sure his brain has been working on this for a long time, and it just finally all came together last night. He will now never forget how to wave and say "hi".
This is both a huge milestone and a turning point. He has started to communicate with us in a way we can understand, and his vocabulary will just get bigger and bigger. The beauty of the brain makes me want to cry sometimes!