SharePoint Portal Server sites can be accessed via the Internet, however, these are the issues that should be considered.
1. It is recommended to use a hardened proxy server (such as Microsoft ISA) on a DMZ zone. This will allow you to have your server hosting the sharepoint app., databases on a trusted (internal) network. Detailed instructions can be found at the following URL:
2. There are different ways of doing this depending on how the domain/network is setup. Compliance negates creating accounts that are only local to the sharepoint environment, as well as authenticating users that come from external sources with, for example, an internal Active Directory. These are NOT options since it would entail exposing a domain member-server or the SharePoint server to the outside world. There are always risks involved when making a system available from the internet. HTTPS (SSL) is a MUST have, and we would also need a certificate (issued by a CA such as Verisign). Also planning and getting ALL the information regarding which network ports etc. that need to be opened in a firewall is very important. In your case if you would expose your SharePoint server to the internet (or DMZ zone) all the ports we would have to open in the firewall would actually make the firewall irrelevant (i.e. it would be as if we did not have a firewall at all).
3. The following setup can be done, however, I would have problems recommending this as a solution. Set up an environment using a stand alone server on a DMZ zone, running basic authentication over SSL (HTTPS). The only thing hosted on your internal network would be the SQL databases using SQL server authentication and only one open port in the firewall to that specific SQL server. This would not be the recommended solution(compliance concerns), as the option of using a hardened proxy is the most secure way it could be setup. It is highly recommended to use a hardened proxy on the DMZ zone with ISA server and SSL certificates(mentioned in the first paragraph).
4. Alternatively, you could use a VPN, which is the way ours is setup now and this is the most secure, and cost efficient way of accessing our sharepoint environment from outside the office. I know that it might be a pain having to do the VPN connection etc, but it would cause you even more problems if your entire network gets hacked. It would not really be a question of If..but When.