If you are browsing with Firefox you probably did not even know that there were video-ad interstitials before many Youtube videos thanks to Firefox Adblock Plus all of them are being blocked. But users of browsers like Google Chrome just had to live with them as limitations in the Webkit engine (Google Chrome is based on) prevented a more efficient blocking of video ads.
But those days are over now: Since the release of version 1.1, the Chrome Adblock Plus extension has improved the video-ads blocking functionality on Youtube. Once installed, users of Chrome Adblock Plus can surf on Youtube without seeing any ad which makes it faster, saves bandwidth, and makes the video-fun much more enjoyable.
Unfortunately, this improvement does not yet effect other video websites like hulu, justin.tv etc. While all banners are blocked just fine with Chrome Adblock Plus, video-ads are still shown. For ad-blocking those websites, Google first has to fix some of the bugs that have been reported by the Adblock Plus community many months ago. Unfortunately, Google does not seem to give a very high priority to improving the code to support ad-blocking extensions.