Now that you have Active Directory integration on Linux, you can authenticate against Active Directory from any application via a simple shell script. One I'm working on intergrates Active Directory authentication from PHP, without all the hassle of installing and using PHP's LDAP extension. Since I'm already configured to authenticate with winbind, I use a command that winbind provides to authenticate AD users, which makes the code required to do an authentication pretty simple.
Recently I undertook the very daunting task of integrating a Red Hat Enterprise Linux 5 server in a Windows Server 2003 RC2 Active Directory environment.
As enterprise networks go, single sign-on between operating systems from all walks of life is sort-of a holy grail of achievement, and with good reason, it was by no means obvious or easy to make it happen! But with some determination, lots of Google searching, trial and error, and a desk to bang your head against, it is ultimately possible.