Properly licensing Microsoft SharePoint is a complex and confusing undertaking, and in our experience, many organizations are either running with far too few, or incorrect licenses. As usual with licensing, Microsoft doesn't make it easy to understand or implement correctly.
Here's an excellent four-part resource we often refer to for everything the basic of what is needed, all the various options and versions, and some excellent specific examples.
For Part IV in my SharePoint 2010 Licensing mini-series, I have put together several different licensing scenarios and detailed the requisite licensing for each. SharePoint is very flexible in both licensing and capability. As with any large company, understanding exactly which licenses you need can be quite perplexing! Hopefully this will shed some light on different scenarios to help you get your SharePoint 2010 environment licensed correctly.
The previous posts in this mini-series include explanations of the different products involved with SharePoint 2010:
SharePoint 2010 Licensing Part I: Foundation, Server, and Designer
SharePoint 2010 Licensing Part II: Windows Server and SQL Server
SharePoint 2010 Licensing Part III: Search, Office Web Applications, and Project Server