The Air Force crew would make the decision to divert. Where they would go would be decided in consultation with the Secret Service.
Sept 11, 2001 showed a lot about how the system works. Though the President and his advisors wanted him in DC as soon as possible, the Secret Service was not going to allow that to happen. And the Air Force flew where the Secret Service decided. The Thanksgiving 2003 visit to Baghdad also showed how the decision to land or divert is not made by the political staff, but by the AF
and the Secret Service.
But to be truthful, I don't think a situation like the Poland/ Russia incident could happen with Air Force One.
The pre-mission planning is more extensive and the aircraft is closely monitored throughout the flight. AF1 is not the only plane landing at that time. There is a press plane which has to land approx 30 min ahead of AF1, with a second press plane due in 30 min after AF1.
Diversion airports are briefed and staffed before the flight takes off. The military aircraft carrying the helicopters and limos come into the destination airport early. There is usually a staff aircraft (B737, B752 or G-IV
which lands right before AF1, and the runway is often checked by Secret Service and a USAF
pilot just before landing. There will be a USAF weather officer on the site for a foreign trip.
If a situation were to develop where a 'look and see' were to be flown, it would be done by one of the other aircraft airborne with AF1, not the aircraft with the President aboard.
In a situation like the Polish crash, AF1 would be able to receive live video from the ground in the communications center. The flight crew - which includes relief pilots and a navigator - would be able to see the actual conditions on the ground.
The US President traveling is a huge circus with between 8 and 25 aircraft involved depending upon the distance and destination. There is almost always a second B747 - the E-4B nearby and for long distance trips, the second VC-25 will usually make the trip, or at least a B757.
All that flying circus is a pain, but would provide many resources to the AF1 crew than the Polish crew had availalbe.
[Edited 2010-06-09 20:37:30]