NEW YORK — Yahoo's last great sales chief and current Martha Stewart Living Omnimedia co-CEO Wenda Harris Millard joined a panel today as part of this week's Advertising Week festivities here in New York. As the talk began, all was light and mirthful. Then MIllard responded to the moderator's question on whether the Wall Street meltdown would spread beyond New York and impact the consumers who fund the advertisers who fund Silicon Valley. Millard's answer: "It's going to be a very tough coming 18 months if not longer. And I'm not usually a pessimist. I see some pretty severe implications for small to medium sized businesses." Millard said "a lot of cash has dried up" and credit simply won't be available to "the average consumer."