One of the new "features" in Android 4.4 - KitKat is the new runtime, which is ART. Which stands for Android Runtime. Previously, Android was using Dalvik from the beginning. So this is a pretty big move, and there are some great things that come from using ART over Dalvik. However, at this point ART is very beta, and is basically only there for Developers. So that developers can get their app ready to use ART in a future version of Android. Well, the folks over at Android Police have come across a recent AOSP Commit that indicates that ART will be replacing Dalvik in the next release of Android, likely to be Android 4.5 or 5.0.
The change is pretty simple. It basically switches ART in as the default runtime, but leaves Dalvik in there in case you need to switch back to Dalvik if you have any issues. As Android Police also points out, that just because this change has been made in the master branch, it doesn't mean that it's confirmed to be the default runtime in the next version of Android. As most coders will tell you that code changes usually do occur early in a project without them making it into OTAs. It's also entirely possible that developers have made this change to do some testing on the next version of Android. Which means it may or may not happen.
Basically what we're saying is that it could happen in the next major version of Android, but it may not happen. It'll depend on how stable ART is, and if the Android team feels comfortable with releasing it in the next version. Basically take this as a rumor. Even though the code change was made, we may not see any differences in a future OTA.
Source: Android Police