Per some informed leaking to the NYT, it seems where Obama is headed. The critical question is whether you can be "Biden" in the countryside--i.e., conduct counterterrorism operations in low-population areas--without the substantial troop presence that gives you the human intelligence generally needed to strike furtive terrorists.
If I'm Obama, the other thing I worry about is the intangible effect of Taliban gains in the countryside. Robert Gates, among others, has warned that substantial Taliban gains within the country--even short of toppling Kabul--would "empower" al Qaeda. And even if we start out protecting population centers, there will be a lot of pressure to expand and push outward as the Islamists expand their reach and use the remote areas as stanging ground for attacks on the urban areas.