Google has obtained a new domain to use for URL shortening. YT.be is now in Google's possession and we imagine that it will be used to shorten links to YouTube videos. Currently Google uses Youtu.be for shortened links that point to YouTube videos, but characters are becoming increasingly important.
YT.be is currently redirecting to Google's goo.gl URL shortening webpage. Google acquired the new URL from a Belgian Internet entrepreneur who originally purchased the domain in May 2003. Purchase price has not been disclosed.
It's unlikely that Google would opt to use YT.be for anything other than YouTube issues for many reasons. The most obvious reason would be branding issues, which is the same reason Google only uses g.co for Google websites.
Google will likely start using YT.be to shorten YouTube URLs soon.