Even with BMW, it's at the discretion of the dealer. Kind of a toss-up. Most BMW dealers will have no problem giving out a loaner IF you bought your car from them. Some dealers will even give out a loaner if you didn't buy your car from them, but they are not as common. Some dealers keep a fleet of loaner BMWs, some just have an in-house Enterprise (or whoever) rep that'll get you a Dodge Stratus.

For an engine replacement you usually get a BMW loaner, though.

So, no matter what kind of car, a good relationship with your dealer will get you everywhere.

-Daver