It completely depends on the dealer. For example our closest dealer is Beverly Hills BMW, and they will not give anyone a loaner...even if we bring in the Z8 Alpina, or if someone has a brand new 760Li that was purchased there.
Last week, when my dealer did some programs I wanted programmed, I was given a 2005 Land Cruiser. Normally we get an E46 325i, and a few times we were given a Z4.