What operating systems are primarily used at the customer organization?
Some contractors don't realize that you can't just go into an organization and change the way the customer does business. For example, if the organization is primarily a research firm that utilizes a strong Unix base, you shouldn't go in trying to do a hard sell on a Windows solution simply because that's what you get the best deal on or because that's what you're most comfortable with. Try to work within the confines of the customer's network architecture and provide a solution that works best for them, not you.
In most cases, however, you'll find a Microsoft authentication solution may work just fine. But there are still a lot of Novell- and Unix-based LDAP shops out there that will need your support as well. Aside from the obvious difference in the operating system being used, you'll also have a number of integration issues related to how you implement the new solution with existing applications, such as email, login software and remote access.