Everything is possible but personally I doubt it.
1. Apple's and Intel's relationship. Apple and Intel have a very strong relationship at the moment. Apple has gotten some special treatment from Intel because of this (early access to CPUs and now Thunderbolt). One of the reasons for this is very likely that Apple uses solely Intel's CPUs. Going with AMD would most likely kill this relationship.
2. Thunderbolt. This is Intel only (at least at the moment) and Apple seems to like it a lot (maybe even developed it with Intel according to some rumors). Apple wants to push it to all Macs. If they go AMD, they cannot.
3. AMD's future plans. While Llano looks great, do the future CPUs look as good? Also, can AMD deliver them on time? One of the main reasons why Apple switched from PPC to Intel was that IBM's schedules were total crap. Llano was supposed to come out in Q1 2011. Now it has been pushed to Q3. While Intel has a solid tick-tock plan and they come up with new CPUs every year, AMD's update cycles are more fragmented. AMD is still using 45nm SOI while Intel switched to 32nm over a year ago. IIRC the current micro-architecture in AMDs is from 2008. Apple ran way from this once already, why would they risk it again? Especially if it is only for one Mac and the gains are not that huge really (it's not like going from PPC to x86 that allowed Windows and all that to be ran on Macs).
4. Drivers. A switch to AMD would require a bunch of whole new drivers. AMDs use totally different chipsets. Apple likes to use the same chipsets and other components in several Macs. 9400M was used in all but Mac Pro. That allows Apple to use the same drivers, thus less work. It also allows Apple to buy bigger quantities and that means lower prices.
Just a few things that came to my mind. Also, we don't have any concrete data of Llano and its specs and performance. The only thing that AMD might offer that Intel can't is a good IGP. That is the only pro of AMD that I can think of.