If you configure the device to not automatically try to connect to wi-fi then the two-button screenlock mode is equivalent to what you think of as 'sleep' mode of the 770. There is no sleep mode, just very efficient power saving and it goes on all the time. Another thing (and this is reported as a bug on buzilla) is that you must also configure the two timeouts (you find them in the screen setting IIRC in the control panel) to be the same value.