A couple of points to add for consideration, don't feel like quoting each previous post. I reference BMW and Mercedes as I drive and have experience with both and consider them to be on the same level (although excelling in different areas). There's a difference in the way a BMW or Mercedes "owner" treats a loaner vs how someone treats a car on a test drive, and then the treatment on rental (Hertz, Avis, etc). Rental car, all care and caution goes to the wind. The fastest car is a rental car right? Demo/test drive hurts me the most when a sales person brings up a new car, cold engine, then encourages the potential buyer to "see what it can do". A loaner generally gets the same treatment as your own car as you probably dropped yours off for service, got in the loaner, then went to work, kids from school, or whatever your weekly routine is. Many say "it was the GM's car" or his wife, etc. You know it's true when they have to find him to get the key. Actually, I feel best test driving one of these when they apologize for it not being "clean" and inside, you see personal effects (ipod, phone charging cable, etc). Then you know the story is true.
My local Mercedes dealer has 2 GL's in the loaner fleet and my BMW dealer has a few 750's. At Mercedes, they're always offering that if you like one of the loaners, it can be sold. At BMW, I have a good relationship w/ the used car sales manager. He has a book/spreadsheet that has all of the loaners, and when each one is scheduled to come out of the loaner fleet and get sold. They go more by that schedule and as opposed to pulling any one out at any time. If you push, they will though.
As for putting cars into the loaner fleet, there is indeed factory support. In fact, it's been rumored that in the end of year sales race between MB and BMW, the latter pushes more support (read:$$) to dealers to put x amount of vehicles into their loaner fleet. Something to do with being able to report a certain number of vehicles as being titled that month, year etc.