Just say no! Friends don't let friends explode cad data! Any other words of wisdom?
If you have to...here is some text from the 2010 help file:
When you import a drawing into Revit, you are importing all the elements, such as blocks and external references (xrefs) from the drawing. (See Implications of Importing vs. Linking for Xrefs.) They are all contained inside a Revit element called an import symbol.
You can explode (disassemble) the import symbol into its next highest level elements: nested import symbols. This is a partial explode. A partial explode of an import symbol yields more import symbols, which, in turn, can be exploded into either elements or other import symbols. This is analogous to exploding in AutoCAD 2010 with nested xrefs and blocks. For example, you explode an xref into other xrefs and blocks. Those xrefs and blocks can, in turn, be exploded into more blocks and xrefs.
You can also explode the import symbol immediately into Revit text, curves, lines, and filled regions. This is a full explode.
NoteYou cannot explode linked files or an import symbol that would yield more than 10,000 elements.
Resulting partial explode import symbols can be exploded again by selecting them and clicking Partial Explode. You can continue to do this until all import symbols are converted to Revit Structure elements.