From Boeing's website.

Freighter Modifications

"Boeing offers the 747-400 Boeing Converted Freighter (BCF), 767-300 BCF and MD-11 BCF, providing the knowledge, experience, certification and support that only the original equipment manufacturer can provide"

However, that's not to say that Boeing and FedEx haven't already come to an agreement for a 777BCF programme but just not announced it yet After all, they (via aquistion of MDC) were brave enough to take on the MD-10 certification weren't they