SharePoint web part deployment error: Object reference not set to an instance of an object error

After deploying a SharePoint web-part developed using VSeWSS I get:
"Object reference not set to an instance of an object error"

Solution: copy the web part DLL to the GAC (it's also possible to register the DLL to the GAC on the batch file 'setup.bat' created by VSeWSS by calling gacutil.exe).