If i remember correctly Bergman first publicly announced about a chip aimed exclusively at notebooks in the start of 2007. So it took 4+ years development for AMD to launch Brazos. Even if we take into account that AMD already got some experience it's very unlikely that new ARM development would take less than 3 years, and that move us in 2015 in the best.
How many players of ARM will stay at that time. Definitely, less than now and those who stay will be more powefull. AMD will look like a niche player.
That new ARM development will also require resources which AMD can take from x86 front only and this will ease its position vs. Intel.
So, i think it would be very big gambling for Rory to play in ARM game. He could take it for BoD satisfaction only.