you should use Option Explicit in your modules so VB doesn't automatically create objects for you. In the line:
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Set idoc = iapp.Documents.Open("c:\SG261LPP-DRAWINGS.idw", True)
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you have gotten a document, however, it is not a specific document object, just a generic document. The SaveAs method uses the file extension to determine what translator to use, but the generic Document does not know about the translator for IDW to DWG.
Instead do this:
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Dim oDoc as DrawingDocument
Set oDoc = oApp.Documents.Open("c:\SG261LPP-DRAWINGS.idw", True)
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Then it should work.
(P.S. - usually in "typical" VB convention, the first letter denotes the variable type. This is why you see "o" in front of many variables for "object". The "i" in front of your variables would indicate to a VB programmer that the variable is an Integer, which in your case it is not. Doesn't make a difference to the compiler, but it might confuse other programmers using your code.)
