Url rewrites for unique site names without subdomains or subdirectories?

Is there a way in multi-site WP3.0 to provide unique site names without using subdomains or subdirectories? For example, my multi-site name is sites-d.ourdomain, but I want the urls for each site instance to look like this:

[site1].ourdomain (instead of
sites-d.ourdomain/[site1] or
[site1].sites-d.ourdomain)

Our clients expect to have the name of their organization followed by “.ourdomain” on every page of their site. Is there anyway I can pull this off? If so, how? I suspect this might be possible with rewrite rules, but my system administrator suspects this may be buggy.

Drupal, by the way, pulls this off by using duplicate CNAME records in DNS.