http://www.cadforum.cz always has some goods tips for Autodesk products. I wanted to point out a new one.
CAD Tip # 4155 http://www.cadforum.cz/cadforum_en/qaID.asp?tip=4155
"The difference between overlaying and attaching an xref is in cases with multiple levels (nesting) of xrefs (A.dwg references B.dwg, which references C.dwg). If you xref-attach C.dwg in B.dwg, and then attach B.dwg in A.dwg, you can see the C.dwg contents/entities in both B.dwg and A.dwg. But if you overlay C.dwg in B.dwg, you can see the C.dwg geometry in B.dwg, but not in A.dwg.
Simply - the overlaid xref displays only one level up in the hierarchy while attached xref appears in all levels up. Overlays also allow to create circular references."