I’ve seen quite a few complaints that AutoCAD 2009 refuses to open some drawings saved in 2000 or 2004 format unless the Recover command has been used on them. Autodesk has now issued a Knowledge Base item about this issue.
There’s no real fix in AutoCAD 2009 yet, just an external workaround. You will have to fix up the drawings in AutoCAD 2008 or TrueView. Either save the drawings in 2007 format or set the system variable 3DCONVERSIONMODE to 0 and then save them in the old format.