I have tried this method and seems to work fine with some hit or misses. You need to to the exact same video with same quality to get to the cached version. I will be using tmg 2010. First go to firewall policy in left pane. Then in right pane you’ll see domain name sets. expand it and go to new domain name set. You will see a box appear type next to name and under domain names included in this set click add. Type *.youtube.com in the new box that appears. Then go to web access policy in left pane and in the right pane click configure web caching. Click cache rules and click new. Call the new rule video. In the next screen click new and go to domain name sets and add video. click next. tick all three boxes. Click nets then yes and next again. Set ttl of objects to 999 and ttl time boundaries no less than 1 week and no more than 4 weeks also click also apply these ttl boundaries. Then click next and the finish. This should work for you your youtube pages and comments might seem a static though. All of the information is merley a guide line you can use your own figures.
