Some that are using the new 2010 have noticed that new Solid bodies folder in the part modeling browser. This tool allows us to specify different features inside a single part as unique solids. Before Inventor 2010 all features that were modeled in a single part existed to create that sinlge part. Mass properties and inertial properties were calcualted on all the geometry in that part file. With the New Solid option inside of the common 3d features like Extrusion, Revolve, Loft, Sweep, etc you can specify multiple solid bodies in one part file, instead of them automatically joining as they have in the past. Also new is the the Make Components command that works great with the Solid Bodies feature. You can use the Make Components command to take all those solid bodies from the single part and turn them into parts of an actual assembly. Imagine a combination of some of the more complex derived component and skeletal modeling approahces that some of us have taken in the past. I am currently talking to a number of users about how this will help in creating some of their more complex assembly designs. For a quick example of the workflow required. Take a look at the video below.
A new type of Level of Detail (LOD) has been added: Substitute. A Substitute is a part file
representing the assembly and is created in the owning or source assembly as a new type of Level of Detail.
The substitute part can be a derived part created from the source assembly. If the substitute part is created in the source assembly by using the context menu selection "Derive Assembly," the part is automatically tagged as a Substitute during the derive process.
When a Substitute Level of Detail is active, all other components are suppressed and hidden in the browser. Significant memory savings can be realized by
inserting an assembly with a Substitute Level of Detail active into an upper-level “consuming” assembly. In the video pay attention to the 
capacity meter watch what using derived assemblies and substitute LOD can do for you.
