Yesterday on 976 MKE-LGA (an A319) we boarded:

1. Preboards
2. Stretch seating
3. "Express" boarding for any passengers who did not have carryons for the overhead bins.
4. Everybody else (not sure if they did this by row or not since I was onboard).

Anybody else experience this? I'm not sure if this made for better boarding or not on this particular flight, but it's an interesting idea.