Being one of the developers for the stable release version of SquirrelMail, it's certainly nice to see it getting some good recognition. However, there are some tiny corrections that should really be made to this article.
The first is mainly the requirements. In your article you stated that you need to be running sendmail, and imapd via inetd. This is incorrect in respect that SquirrelMail was designed to be as flexible as possible and will work with any IMAP server, and SMTP server. This means that you don't need sendmail, you could use postfix, or qmail if you wished. You can also use Courier-IMAP as an alternative for example. Because of this, it may not necessarily mean that your server needs to use inetd to run, and could run as a stand alone daemon.
Another small correction on the requirements. You don't necessarily have to be running apache. Apache is probably the prefered method, but PHP is available for IIS, PWS, and some other web servers as well.
The second is method of authentication. The articles says that SquirrelMail uses /etc/passwd. This is partially incorrect as it really depends on how the imap server authenticates. For example, the University of Washington's imapd (referenced as a requirement) uses /etc/passwd, while courier-imap can use a number of different methods from userdb to mysql databases, and even ldap or PAM.
As with authentication, where the article says it polls mail is also incorrect. That is totally dependant on where you tell your mail server to store it. Most mail servers allow modifications to where files are stored.
Now for a little information. :)
Some things that aren't mentioned about SquirrelMail are it's functionality. Apart from being a simple mail client you can use to send and receive mail, it has support for plugins. With plugins you can extend the features of SquirrelMail itself, for example POP3 collection, or SpamFiltering. There is even a nice little plugin for use with your spamcop account, allowing you to submit with a simple click of a link.
Don't be alarmed by the screenshot either. That isn't how SquirrelMail looks. SquirrelMail actually has the ability to allow the user to change colours for their own preferences. You can also alter the size of fonts, colours, and such.
pmccann asked how passwords are sent. Just like any web based form submission they are normally sent via plain text. This is fixable by installing your webserver with SSL support and allowing logins to SquirrelMail from the secured login page. There are plenty of documents on how to setup apache with mod_ssl flying about (even on the mod_ssl page too).
Currently developers are working on a method of support cram-md5 and digest-md5 authentication for IMAP servers so that we reduce the amount of plaintext data that is sent between points.
Further information about SquirrelMail can be seen at the website, http://www.squirrelmail.org, including some basic installation guides. There is also the squirrelmail irc channel on the openprojects network (now freenode) on irc.openprojects.net in the channel #squirrelmail.
Thanks to the author for the original article.