YouTube clips have previously allowed for captioning, but it was a system that was a little buggy and only worked from YouTube's site. That's all changed with the advent of machine-generated captions and auto-timing.
Google's automatic speech recognition (ASR) technology has been paired up with YouTube's caption system to create automatic captions for the majority of the videos that are uploaded (at the rate of 20 hours of footage every minute). It should cover the needs of your caption viewing needs, although they'll continue to streamline and improve this technology as things progress.
If you don't want to leave the captioning of your video to the hands of the YouTube caption system, they've also added an auto-timing function, which allows you to upload a simple text file to be read. The text will sync up with your video without any timing breaks or coding added to the file. The previous system of captioning was mainly user-generated and required a little professional know how when it comes to coding and video editing — thus, most videos were caption-less, making them inaccessible to those with hearing impairments.
For those who aren't in need of captions, there's still an added bonus for you. With the advent of all the new text in association with what's on screen, it will make videos easier to search for and find, even if you don't know the title.
Automatic Captions in YouTube [The Official Google Blog]