[Update 10/11/2012: What a relief! All my old YouTube comments appear to have been restored successfully. Thanks to the YouTube team for correcting the situation so promptly.]
[Update 10/10/2012: Some news: I received an email from the youtube team saying "A small number of YouTube users may have had their comments deleted as a result of an error on our part". They sent a link to a reinstatement form, which I submitted, and I now wait with fingers crossed for a speedy and successful recovery. Here’s a link to that form, if you feel you’ve also been affected by this issue!]
Today I noticed something completely depressing! I was looking back at some older comments on one of my videos and I noticed that none of my comments were there. When I clicked on someone’s reply to one of my comments “in reply to John Cooper (Show the comment)”, it popped up a dialog saying “The comment no longer exists”.