I don't think there is anyone on the scene right now or in the pipeline who can break 2 hours, in fact I don't think it is going to happen for a very-very long time. Someone who is fully committed to the marathon, but has the track speed to run a sub 26:50 10Ks and consistent 58x halfs has the capability to run a 2:02.
I think in 2005 when KB won the WXC long and Short, ran 12:40 in the 5K and 27:17 in the 10K, I think he could have run around 2:02 if he adjusted his training for it. Sammy was heading towards a 2:02 in my opinion if he could have maintained his focus. Sammy ran a 2:06 solo in 78 degree temps and on a course with sharp turns and won Chicago in 2:05 when he wasn't fully fit. Both of these guys were only in their mid 20s when they were at the top of there game and perhaps that's when we could have seen a 2:02 marathon. - The 2:03s that we are seeing is about where the marathon should be, to be in line with other events, but a sub 2 hour marathon would have to be someone better than Samuel Wanjiru and Kenenisa Bekele, which kind of makes the whole discussion kind of silly when you really think about.
Hall's 59x half marathon from a couple of years ago say's a 2:05 might be in his future, a 2:04 is not out of the relm of possibilities, a 2:03 is doubtful, a 2:02 is highly unlikely and a sub 2 hour marathon is absolutely impossible.