There is no option to disable inline playback permanently. You can switch it off, but YouTube WILL turn it back on. Same with autoplay I've heard. The worst part is anything you mouseover for more than a second or 2 will be added to your history. You know that horrible music video YouTube really wants to show you? if your mouse accidently hovers over it, it will go into your history, and YouTube will use that to recommend more horrible videos to you. If you've ever been stuck with some irrelevant topic taking over your recommendations, this is probably why.