I was unaware of this until today: http://www.damninteresting.com/ww2-america-warned-hiroshima-and-nagasaki-citizens/
Do we know if many people heeded these and fled to the country?
Nobody in Hiroshima and Nagasaki did, because if you can actually read the flier you can tell neither of those cities is on there. The dropping of leaflets warning civilians beforehand occurred as part of the regular bombing campaign -- as far as I remember the US strategic bombing survey doesn't mention whether or not people fled as a result, since it's interested mostly in whether or not the warnings appreciably lowered the destructiveness of the conventional bombs (they determined it did not).
So no, they didn't flee, because those leaflets warning SPECIFICALLY the citizens of Hiroshima or Nagasaki were never dropped (indeed, Nagasaki wasn't even the intended target for the second bomb -- Kokura was).
After the first bomb was used leaflets and broadcasts describing it in Japanese did start to go out, but as far as I know neither Kokura nor Nagasaki was targeted specifically with them.
Edit: Helpful threads for reference! http://www.reddit.com/r/AskHistorians/comments/2z7wt6/ive_been_doing_a_lot_of_research_and_cannot_come/