Alright, let me try to clarify what you are asking and hope I can give you the answer you are seeking.
One single NLM service can run all the different versions of your Autodesk software. That means the license file can have AutoCAD 2006, 2007, 2008, etc. Generally each license is simply appended to the next into one license file and when done correctly the NLM will handle all available products.
The version of the NLM is important in this case, however. The version needs to be at least the latest version of the product that you are using. Therefore, if you are going to use 2008 licenses, you need to install the Network License Manger from the Autodesk 2008 media.
Now, starting with Autodesk 2009 products and NLM, Autodesk has issued a new license package feature. This allows your existing AutoCAD 2009 license to also run licenses for prior versions going back to 2006 (all with one license.) This eliminates the need to constantly append your old license Increments to the new license files each year.
Sound good?