From Fedora Project Wiki
Will this support USB flash drives?
USB key's are up to 32GB these days which makes them much viable choice to use for containing packages even whole repo's than optical media. ( http://download.fedora.redhat.com/pub/DIRECTORY_SIZES.txt )
and how to make a usb volume into a media repo? ie. it should contain a media.repo file having something like
[InstallMedia] name=Fedora 11 mediaid=1243980279.769793 metadata_expire=-1 gpgcheck=0 cost=500
and a .disc info file which contains something like
1243980279.769793 Fedora 11 i386 ALL
this is beside having repodata [generated from createrepo]
so what do you think ? does it worth the effort ?
note: you could be interested in yum-plugin-tmprepo