Typically in the mobility projects I build I'm targeting between 50 and 150 users.    Although there are some expectations to this project size, this is normally where I'm at.   So to target this user size,   I need to have a server infrastructure comprising of -

Exchange Server- for messaging
ISA Server - for Internet connectivity and firewall.
Sharepoint Services - always comes in handy.
SQL Server - got to be there to hold all that lovely line of business data.

Now at the 50-150 user count that list becomes an ' expensive ask' if not already in existence on a client site.     Of course its rare that the client is a complete greenfield with non of the above but it does happen.

So for the past few installations we've been using Small Business Server 2003.    Its great!.    All of the above products (with the premium version) come in the box.

The best bit, is that a 2 DVD install and its running configured, locked down and ready for action.    Just add the SQL Server Compact Server tools and your ready for line of business action...

Now I'm the wrong person to discuss user CALS and such like,  but from a developers perspective it all works like a charm.   It means I can concentrate on the application and not the plumbing.   

All the product information is here -

http://www.microsoft.com/windowsserver2003/sbs/evaluation/faq/prodinfo.mspx