Have we seen the last significant experiments in the configuration of jet airliners?
With current fuel prices: Yes, we have arrived at a mature design which will not change much. Recent trends like the increase in aspect ratio and engine bypass ratio will continue, but the general arrangement will remain unchanged.

Just look what happened in the first 50 years of airline operations, and how much changed after that in regards of aircraft configurations:





If fuel prices rise to 200 USD per barrel and more, expect slower airliners with less sweep, much higher aspect ratio and probably even braced wings. To enable bracing the wings will be mounted on top of the fuselage, like on the Hurel-Dubois HD.31.



Boeing SUGAR Freeze concept. Note the third engine in the back which ingests the fuselage boundary layer, thereby reducing separation and pressure drag.

While you might read about two more new design directions, a supersonic airliner and the blended wing-body (BWB), I am rather skeptical that any of those will be adopted in the future.