With Android 4.4 KitKat, Google introduced a new experimental runtime dubbed Android RunTime (ART).  Even though the company did not make much noise about it then, it was a given that sooner or later Dalvik would be replaced by ART. According to some commits and comments in AOSP from Google engineers, it looks like the change is set to occur with the next version of Android.

ART has significant advantages over Dalvik and can play a major role in making Android apps work as fluidly as iOS apps. Even in its current experimental stage, there is a noticeable improvement in switching between apps and opening/closing them while using ART.

Dalvik is dead, long live Dalvik! DO NOT MERGE  — Google engineer Brian Carlstrom

Most Android OEMs have already updated their system UI framework and apps to work with ART. Android developers also are slowly and steadily working on making their apps compatible with ART, preparing themselves well in advance for this inevitable change. (Adipex)

The above comment from a Google engineer and the merging of the commits are as official as things can get, before Google publicly announces this change at Google I/O next week or whenever it decides to release a new version of Android.