Ready, this is real complicated......
Change the template of the users mail database file to iNotes6.ntf on the server. Set a password in the person document for the internet password.
If you have HTTP enabled. Voila! You have iNotes.
Now then...
Before you just open this up for access... ensure that names.nsf cannot be browsed anonymously, nor by default... (Check the ACL!)
Turn off directory browsing.
Decide whether or not you want to use "More Names with less security" or "Fewer names with more security". The first allows you to use the shortname or the full user name, the second allows you to only use the full user name...
If you have multiple servers, enable SSO.
Change the timeout settings so that the web connections don't remain active forever.
Make certain that the server is allowed to run Java agents.
Set up a policy that allows the password to by synced with the internet password. Warn the users. Expire all of their passwords. When they reset them, they will be in sync.
Consider setting up an ISA server to do a reverse proxy to the Domino server. You will be amazed at how simple this is to set up and lock down. You can go to isaserver.org for advice and articles on setting up a reverse proxy server for http sites. I have an ISA server running basically through three firewalls (not my choice, I work for the datacenter of a bank) and the performance is outstanding.
This is not a hard thing to implement or maintain.
I would highly recomment that you get the Login redirect database template off of the sandbox (notes.net). It has a very cool and very easy to set up method of authenticating users... even across multiple servers and through reverse proxy.
redbooks.ibm.com has a great book on setting up iNotes.