To answer your question first, if your AutoCAD 2010 is from the Revit Suite (meaning that your vanilla AutoCAD was installed from Revit and not from it's own separate media) then it will take a Revit 2010 license. This means that on one machine, if you launch Revit 2010 and the AutoCAD 2010 from that suite, it will only use one (1) license.
If you launch Revit 2010 and AutoCAD 2009, it will consume two (2) licenses.
I do not have anything to post to explain it, but here is basically how it works:
If a user on one machine launches two versions of the product, it will use one license when a) The product is the same version (Revit 2010 and Revit 2010) b) The product is from the same installed suite (Revit 2010 and AutoCAD 2010 from Revit.)
If a user on one machine launches two versions of the product, it will use two licenses when a) The product is from different versions (Revit 2010 and AutoCAD 2010 installed from AutoCAD 2010 media) b) The products are from different release years (Revit 2010 and Revit 2009.)
The only time the product uses a single license for multiple products is when it is essentially the same product. Any difference in the installed license or license years will require an additional license to run.
Make sense?