user 'tagged' to domain through which they register..?

It may. But only if you are using advanced editing options described in 6.2.

It only affects how users post content if you have a certain configuration as described in 4.2.2 Content Editing Forms. Specifically:

  -- Take user to their assigned domain
  Before being presented the editing form, users will be taken to the
  first domain assigned to their user account.  This function is most useful
  when you users are only allowed to enter content from a single domain.

There may, in fact be a small bug, since the routine that controls this behavior doesn't check to see if the target domain is active. Though, if it is not, then the user will be directed to the root domain anyway.

So the end result would be: if user two registers through user one's domain, and user one goes away (or is blocked), then user two would be redirected to the primary domain when posting. But, again. this only occurs under one possible configuration.

So I think this is not an issue.

You, as the admin, can also re-assign users to other domains.

It might also be desirable to force users to sign up from a certain domain -- but that is a feature for another day. See http://drupal.org/node/278582 for the current workload.

Hmm.

This code (in domain_form_alter) probably does need to sanity check itself:

          case 2:
            $domain = domain_lookup($first_domain);
            if ($domain['domain_id'] != $_domain['domain_id']) {
              domain_goto($domain);
            }

See attached. Accessing an invlaid domain on node creation now takes you to the default domain.

Committed to HEAD and 5-dev.