This is something that I highly recommend for people here that are going out for a photo week or weekend and have a DSLR body. If you are going to rent for a week, you can rent lenses from
www.lensrentals.com and have the lens for a whole week. The best thing is that if you live in a rural part of the state they ship it. Apparently they Fedex it to you and it works well from what I hear. The prices are good, for example a 300mm F2.8L IS is $174 for the whole week. That's about $35/day. Well worth it if you want to take some amazing tack sharp images on a vacation or trip.
Your other option is
www.glazerscamera.com for rentals. They rent by the day and they are more expensive. You also get Sunday free essentially. If you are going for a weekend and are near Seattle this is a good option. That same 300mm F2.8L IS is $40/day so it's still reasonable.
I also found another site
www.borrowlenses.com but haven't heard anything about them.
I've found that I would have a hard time locked in on one type of telephoto lens. I need different lenses depending on where and what I'm shooting. Instead of investing in 10k+ in glass I've rented for the last 3 years and it's worked great. Additionally, when they come out with a new version and upgraded features I don't have to sell my lens and buy another. I also can't get out to shoot that often. If I shot every weekend then it wouldn't make financial sense to do this. For the guy that goes on 2-3 photo weekend/safaris per year it works great. It's just an option I thought people would like to consider. If you have never used a big pro lens they are fun to try and make that Glacier or Yellowstone Park trips unforgetable.