Embedded Linux has a large memory footprint and generally requires a MMU Memory management unit....in the Megabytes - region. (check uClinux that doesn't require an MMU - but still substantial memory is required)

There are many advantages in using Linux - specially in the operation system and code function reuse.... but not particular cost effective when compared to simple 80c51 applications.

There are quite a few ARM9 cores which sport a MMU and external memory controller for the "large" memory requirement.

Also, Yes as above there are many flavors of Linux and some may be particularly tailored to the micro controller (really a microprocessor of choice).

Because of the large memory requirement - often Microprocessor - rather than standalone micro controllers are used.

Because external memory mapping is required for the large memory - often "flavors" of Embedded Linux target specific Linux development boards...

Here's an example http://www.embeddedarm.com/

The term BSP (board Support package) is the term used in getting the embedded Linux board up and running with the BSP code from the board manufacturer...

So yes - hence there are variation on Linux BSP's depending on the Linux embedded hard chosen.

Joe