I'm hitting this too. It was working in beta 1, but not beta 2. I'm only testing with the simulator right now.
I think this is a bug as well, but haven't submitted a radar yet. While experimenting with beta 2 I'm using John Susek's workaround of implementing onEditingChanged in the TextField constructor and checking for a "false" argument (meaning editing isn't currently taking place) as a location to handle the normal onCommit activities.