Is it that OS X .6 specifically supports 1.44MB floppy disk drives, or that it supports USB removable FAT volumes, of any size? I would think the USB drive itself would be responsible for the MFM encoding scheme, leaving it to send/receive the FAT file formatting via USB. As long as OS X understands FAT and allows 1.44MB volumes to be read and written, I can't imagine any Mac OS would ever omit the USB floppy disk – and I don't see FAT being eliminated anytime soon. This goes for any USB device. It's like asking how much longer will Apple support HFS, which can no longer be used as a startup disk?
Unfortunately, Apple dumped MFS with OS 8, and never fully revived it. Even MFSLives runs only on Tiger. I really wish someone would update MFSLives for Leopard as well as enabling read & write capabilities. Then if someone would only develop that USB GCR floppy drive!