Trying to give something back to the Community...
This post made by Brent Dunsire over on the The Configuration Manager Support Team Blog is well worth a read:
“In this post I'd like to share information gleaned from a quarters worth of support incidents as regards Configuration Manager 2007 Client Site Assignment. The goal is for this post to help by providing details on common problems driving calls to support.
It is worth noting here many of these details will assist in Client Installation failures as well even though that is a topic for another day. It is also worth noting that many of the problems perceived to be Client Installation failures are in fact successfully installed clients which are unable to assign to a site. In the Admin console when the property Client = No, you may have a successfully installed client that like E.T. cannot phone home. Actually, unlike E.T. it does not yet know where Home is...”
Read the complete post at http://wmug.co.uk/blogs/cliffs_blog/archive/2009/05/20/configmgr-examining-some-common-problems-with-site-assignment.aspx