AutoNumber vs. Natural Primary Keys

Dear all,

As a relative newbie to the art/science of database design, I was wondering
if there was any consensus view on whether it's better to use an AutoNumber
field as primary key for a table rather than select a (human-understandable)
natural key, assuming one exists that you can guarantee will be unique.

Does the situation change between a 'main' table containing entity data
(tblCustomers, e.g.) and a lookup table you're using to store values for a
combo box?

For that matter, is there any situation where you might prefer to use a
random AutoNumber instead of an incremental one?

Any ideas and advice very much appreciated.

Thanks,
Alex.