Yes, they are allowed. The steward/ess will announce when it is save to turn on computers and small electronic devices.
The weird question is bluetooth and other wireless networking standards. You can not have any item that broadcasts on an aircraft because it is the broadcasting of a signal (cell phone) that can interfere with the electronics of an aircraft.
I know the p800 from Sony has a radio-off function that will allow you to use its PDA functions on an aircraft (it turns off the cell and the bluetooth), but you will need to, in the future, read your manual about whether a bluetooth device can have its transmit/receive functions turned off for use on a plane.