"Later this year we'll launch a new feature on YouTube's mobile apps that will help you reach fans -- even when they're not connected to the Internet. This upcoming feature will allow people to add videos to their device to watch for a short period when an Internet connection is unavailable."
AllThingsD reports that this feature will be available next month and that the downloaded videos will be available for 48 hours. "Next month, Google's video site will let viewers save clips on their phones and other mobile devices for up to 48 hours, so they can watch them when they're not online. The videos will still be free, and Google will run ads on the clips, which will be available via its mobile apps."
YouTube's Android app already has a preloading feature for subscriptions and the "watch later" playlist, but this feature has a catch: you need to be online when starting to play a video. This makes the feature less useful.
A few years ago, YouTube experimented with downloads for some partner channels. Now it only lets you download your videos. There are a lot of third-party applications and extensions for downloading YouTube videos, but you won't find them in the Play Store or the Chrome Web Store because they break YouTube's terms of use.