I have not heard anything about it, but judging by the delay in USB I doubt that it will happen without a major change in the way Mach 3 works.
Mach 3 essentially hijacks Winders, running its machine control loop at roughly 25 KHz or so. To run USB you would probably have to build a dedicated controller board and pass the toolpath over to it.
I don't usually read the Mach forum. Is somebody pushing USB, or are you just not wanting to use a parallel port?
- Just Gary