I will be participating in an ad:tech panel this afternoon with the title “Digital Economy 2010: A Robust Recovery or Another Year of Trench Warfare?" I have to admit that I am not entirely sure what that means, but in a prep call with moderator Mark Mahaney we came up with some fun questions. The one that is most interesting is whether we may be facing a long-term decline in the percentage of GDP that is spent on advertising. I am pretty sure that I saw a statistic to the effect that adverti...