I've raised Bug #943 to address the lack of a fast and efficient "standby" mode - the use of the power button makes the process too fiddly to activiate in a hurry and the default operation ("Lock touch screen and keys") leaves the touchscreen active for a few minutes until a timeout kicks in (which of course it never does when the device is bumping around in your pocket).

Please vote on it or suggest other alternatives/enhancements.