Provider-based DNS naming?

Piet Beertema <Piet.Beertema at cwi.nl> writes
 *     I.e. is there something that can be done to disuade a provider - say
 *     provider.top from registering his customer companies as
 *     	company1.provider.top
 *     	company2.provider.top etc..
 * The rules for registering a domain under .NL explicitly
 * forbid this (short-form translation: "subdomains under a
 * domain registered by an organisation may only pertain to
 * said organisation; registering third parties under the
 * domain is not allowed").

Hmm, does this only apply to real domains or also to hostnames which
is very usual practice for dialin/SLIP type of providers? It basically
comes down to the same thing. 

company.provider.top	A	a.b.c.d

or

company.provider.top	NS	some.where.else

in my view is kind of the same, at least in binding the companies
domain name and email address to the provider. Even in NL the first
one is common practice.

-Marten