Or is this just something that will be done for us when our phones get upgraded to Android L?
Which would be after we're upgraded to 4.3.3, 4.4.4, etc...which, I'm thinking, could be the proverbial '12th of never.'
ART should provide better performance and power usage, at the expense of some delays when rebuilding the cache (which it does the first time it boots after switching to ART or if you clear the cache in recovery), and when you first install the app and higher storage usage. I guess the simplest way to look at it is that with Dalvik, the OS compiles some parts of the application every time it is launched. With ART, it pre-compiles some of the stuff when you install the app, keeping it around permanently (until you clear the cache or uninstall). ART will offer better performance and power consumption.
That being said..... I wouldn't worry about it... Not all applications are optimized to use the different runtime, and some, like Xposed, are totally incompatible at the moment. There's no compelling reason to switch at this point. When the next major version of Android is released, it'll handle all that for you.
Though, I pity anyone carrying a few hundred apps on their phone when the boot that first time.