"China, of course," the WPP Group chief executive snapped. "Alongside the internet, China tops the list of things I worry about."
Sorrell and his contemporaries have been talking about China for some time. Indeed, its importance to the global communications industry is one point on which the Tempus Group founder, Chris Ingram, and Sir Martin are likely to agree (read Ingram's feature on how China's brands will go global, page 32).
But for all their excitement about China, both know that the incumbent government isn't going to roll out the red carpet for international businesses without having a hand in their operation.
Yes, international trade regulations have relaxed. Yet Rupert Murdoch and co (feature, this page) can only stand by and watch the "bamboo curtain" being lowered for every industry except for media - such is the unwavering desire of the Chinese government to control what information its people can access.
It's hardly surprising that, in their pursuit of China's billion-strong audience, the likes of MSN, Yahoo! and now Google are bowing to censorship demands. And by doing so, critics say, are betraying the principles of free, democratic information-sharing on which the internet was founded.
But the realities of operating in China are sinking in. In his book, Billions, the veteran adman Tom Doctoroff, JWT China's chief executive, observes: "Distribution death-traps, kamikaze sales teams, infectious corruption, and seemingly indecipherable regional preferences - not to mention 800 million rural-dwellers living on less than three dollars a day - have shattered the illusion of gold in the topsoil."