On AA flights there is usually a set of chimes at around 10,000 feet, which is usually followed by confirmation from cabin crew that it's ok to turn on electronic devices. That's helpful.

(On the other hand, it's one of my pet peeves that the seatbelt sign on AA flights stays on for at least the first 45 minutes of the flight and comes right back on at the merest tremble of turbulence; that's presumably because of worries over liability claims...).