A slightly modified version of ScottF answer, which does not require absolute file paths:
if WScript.Arguments.Count < 2 Then
WScript.Echo "Please specify the source and the destination files. Usage: ExcelToCsv <xls/xlsx source file> <csv destination file>"
Wscript.Quit
End If
csv_format = 6
Set objFSO = CreateObject("Scripting.FileSystemObject")
src_file = objFSO.GetAbsolutePathName(Wscript.Arguments.Item(0))
dest_file = objFSO.GetAbsolutePathName(WScript.Arguments.Item(1))
Dim oExcel
Set oExcel = CreateObject("Excel.Application")
Dim oBook
Set oBook = oExcel.Workbooks.Open(src_file)
oBook.SaveAs dest_file, csv_format
oBook.Close False
oExcel.Quit
I have renamed the script ExcelToCsv, since this script is not limited to xls at all. xlsx Works just fine, as we could expect.
Tested with Office 2010.