HTML5 introduced the
Both async and defer scripts begin to download immediately without pausing the parser and both support an optional onload handler to address the common need to perform initialization which depends on the script.
From the webkit blog, so the behaviour is not necessarily the same across all browsers. So, performance would be better if the scripts are still at the end, as they will be downloaded later.
Edit 2017: browser support is now much better, so you can probably get away with async/defer scripts in the head. It's still probably a safer choice to put them at the bottom; new browsers will still download them early even if they're not in the head.