The newest mobiunpack can split the MOBIs produced by the newer versions kindlegen into its stand-alone MOBI and KF8 components. The resulting KF8 file is the same* as what you would get delivered from Amazon.
Just run the gui (Mobi_Unpack.pyw) and make sure the "Split Combination Mobi Files" box is checked.
* The same as far as testing has proven so far. Amazon could always change things at the drop of a hat.