Yesterday (Tues) were playing at Putney Park with Jodie & Hannah and the kids. 

Eli took off climbing the stairs, and met a lady with her son of approx. 1-2 coming down the other way.  They eyed each other for awhile and then I hear a distinct "Hello!" and didn't think much of it.

Until the lady said "Did he just say hello??!!" to which I replied "Oh I guess so, I just assumed it was yours as he's a bit older!"   (Although I was facing her son and didn't see his lips move so I guess I just hadn't thought it through...)  

He also waved and said "Bye" to Amelia when I was putting him in the car at the end of the play.  

Won't do hello or bye on command but will when he really wants to!