Jason,
Be sure to take weight into account, the Nikon 500 weighs 8.6 lbs, the 600 weighs 11.2 lbs. After taking the BIF workshop from Jim Neiger and Doug Brown my wife and I could handle the weight of a 500 but not a 600 hand held. Even with a tripod the extra weight still has to be carried around. If you're serious about buying then do some reading about Nikon's production problems -
http://www.bythom.com/2010%20Nikon%20News.htm - look for "Availability Predictions" then the last bullet "Exotic telephotos" - look up Nikon lenses at B&H, sort from most to least expensive, and count how many aren't available. If you're renting then no issue. But even the used market is tight because there are so few new lenses being made. Lest anyone think I'm anti-Nikon, my wife and I shoot D300 and D700, own a Nikon 200-400 and Sigma 300-800. Love what we have, are not happy about Nikon's long lens behavior. We're seriously thinking about jumping to Canon when it is time to replace our bodies, no availability problems and judging from the new 400 f/2.8 they are going to cut several pounds from the 500 and 600 next year.
I agree with Randy about shipping, both for rental and owned lenses. On a trip to Florida we shipped the 300-800 and tripods FedEx to a 24 hour FedEx Office (ex-Kinkos) that was near our hotel. You can set things up to have shipments held for you with both FedEx and UPS, FedEx has more places with late night hours.