The Canon Digital Learning Center
has published an article
on the just-released Canon EOS-1D X
firmware version 1.1.1.
As I expected would be Canon's position on the question, the Canon EF 70-300mm f/4-5.6L IS USM Lens
is officially not compatible with the Canon EF 1.4x III Extender
As discussed here
, the 70-300 L was initially included in the 1D X's f/8 AF-capable lens and extender combination list on the new Canon 1D X special firmware page
. This lens is no longer included on that page - and is not included on the CDLC firmware in-depth page's compatibility list.