It's no secret that everyone believes the mobile advertising space is going to be huge. Massively, massively huge. If you need any proof of this, just look to Android. That's essentially the entire reason it exists. In fact, it could be a $3 billion business by 2014, MoPub CEO Jim Payne believes. Not surprisingly, the investors are right there with him in that belief. MoPub, a service which can most easily be thought of as "DoubleClick for mobile", has just received a seed round of funding from Accel Partners and Harrison Metal Capital. The ad-serving platform for mobile apps and websites is an obvious bet as both Rich Wong of Accel and Michael Dearing of Harrison Metal have been heavily involved in the mobile space, with companies like GetJar and AdMob.