Taken about five minutes after Neil Gaiman walked up to Scott McCloud, introduced himself, and asked, "What's all this I hear about a 24-hour comic, then?"
I took one look, and thought, "Twins!". They even gave me identical "well, ok, if you want a photo of us, but I don't know why you would?" looks.
Date's an approximation, but I'd assume it was Thursday of the con. This was definitely Comic-Con 1991. Scott's badge says so.
7/29/06 Addendum: Ivy and Neil have been comparing dates, and this was NOT the first time these two met; at this point they were already friends and had played Five-Card Nancy together. Oh, well, still a good photo of them. :-)