Informação retirada do blog Between the Lines
"...AutoCAD 2008 Compatibility
AutoCAD 2008 uses the same DWG format as AutoCAD 2007 and it can read all DWG files from AutoCAD 2 back in the early 1980's.
The API is binary compatible so 2007 applications should run fine or check with your 3rd party developer. 64bit is new so any ARX you want to run on the 64bit version of AutoCAD will require recompiling.
Here are some compatibility highlights from a AutoCAD 2008 Questions and Answers document. ..."
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Resumindo, poderá haver problemas na versão do AutoCAD 2008 64-Bit em relação as aplicações add-on.