With the release of Autodesk Inventor 2012 Service Pack 2, Autodesk has changed the installation requirements and behavior for Inventor service packs going forward: Service Pack 1 is no longer required to be installed as a prerequisite for the installation of Service Pack 2.
Service Pack 2 includes all fixes included in Service Pack 1. If you are updating from the release version of Inventor 2012, you only need to install Service Pack 2 to obtain the latest updates.
Uninstalling Inventor 2012 Service Pack 2 uninstalls Service Pack 2 only and maintains Service Pack 1 and/or any Hotfixes you may have installed previously.
This is the new standard for Inventor Service Pack behavior and will hold true for any subsequent service pack releases for 2012 and 2013.