Why would they even wanna open up internationally? the answer is simple! the world goes way beyond the USA, the internet its a global market, there is no way a huge company like yahoo would succeed on this type of program without going International.
for example, you think a company like Apple computers doesnt have the need to advertise outside the USA? But forget about huge corporations, the small companies/website owners have this need to advertise outside the USA also.
the thing here is that the problem is on the Overture system. they dont have the option to target the traffic by country yet. I think they are working on this feature before they go global