This may also be a versioning issue. If you are opening a drawing that was not created in the current version, there can be problems opening the files. The safest way I have found is resaving the drawing info the current version before it is opened.
This is done using objectDBX or resaving the Database if your using .net
This was discussed here
http://discussion.autodesk.com/forums/thread.jspa?threadID=658684
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