I can only speak for what I know occurs here in the USA. When a flight is told to switch to another frequency ("N123PQ contact Boston Center on 126.22"), the pilot should read back the frequency given and then switch over to the new frequency. There should be really no delay in doing this since the next controller is already expecting the flight to check in on his frequency. And this is true regardless if the flight is at cruising altitude, descending or climbing, or on the ground taxiing. I would think that this also would be true anywhere else in the world. If there's a delay in the flight checking in on the next frequency, the controllers will more than likely start to try and figure out what's going on (calling the flight on their frequency and/or asking the previous controller to check if the flight is still on the previous frequency). It does happen occasionally that even with a read back of the frequency a pilot will still accidentally dial in the wrong frequency in his radio. In that case the pilot will eventually go back to the previous frequency when he realizes there's no answer.
As far as the Malaysian flight goes, there's an extensive discussion going on in the Aviation Accidents/Incidents Forum: http://www.liveatc.net/forums/aviation-incidents/malaysia-airlines-reports-atc-loses-contact-with-mas370-wmkk-zbaa/