The only way to do this in SPS 2003 would be to enable Anonymous Access.
In SharePoint 2007 you can setup alternate non-domain authentication,
but not in 2003.

I've never used SPS 2003, so I might be way out in left field. Could
you set up local accounts on the WFEs for non domain  users? I've done
that for limited situations with WSS v2 and it worked fine.

You can use local accounts on the WFE's, but if you have multiple WFE's
they will be viewed as different accounts even with the same username.
Local accounts only work reliably if you have only one WFE.

In the instances that I've used local accounts it's been with four WSS
v2 WFEs and it worked fine. Of course I had to give each of the four
accounts access  to the content, but that wasn't a big deal for what I
was doing. Again, this was WSS only, so SPS might have some features
that keep this from working well.

Could ADAM be a quick solution for this? Can you do ADAM non-FBA?

No, ADAM isn't supported on "v2 (WSS 2.0 and SPS 2003) " it requires
v3.

I don't know if you can do it in a non-FBA manner. I've always done it
using the AD LDAP Membership provider in 2007. But I do remember reading that
it couldn't be used in SPS 2003.