As to andrewc's question, I've done a quick calculation, and based on a static line jump from 10,000ft, the parachutist will be in the air for 6.6 minutes. A 10 knot wind will blow them 1.1nm downwind in this time.
If we assume that parachutists don't jump if the wind is much greater than 30-40 knots at height, leaving a lateral distance of 5nm should give a margin of safety under most conditions.
I'm assuming that above 10,000ft, sky-divers will be free falling, so falling at much greater speeds, and will be blown off course much less; so I'd guess 5nm would be good for most situations.
Anyone got any better information?