I've seen on the newsgroups that a few people are having problems publishing sites with v3 externally. They do the standard thing, forward port 80 on their router/firewall and it should work. Unfortunately they find that external users are redirected to the internal name of the server and it fails. This is because v3 introduces some new functionality for external access. To make the external access work the way you want you'll need to create an Alternate Access Map to the external URL. Go into Central Administration, Operations and click Alternate Access Mappings under Global Configuration. It will take to you this screen:
Larger version available here.
Click Add Incoming URLs and add the Internet facing address and your site will now work externally as well as internally. You'll probably want to assign it to the Extranet zone. Assign an outbound URL as well. That will allow WSS to use the correct URL when it sends out Alert URLs.
Update: This has changed some since Beta2, which is what this post was based on. Please see my updated post here.