Homebrew PCBs

> 
> I don't believe the USB interface is capable of fast enough throughput
> to handle direct stepper phase control, but step-direction commands
> might work. (flow control would still be an issue).
> 


I bet it is. The widely available FTDI USB to serial chips will do 250kbps, just how fast does it need to be? That said, I'm not sure why you'd want to do direct phase control, there are some nice stepper driver IC's out there. I played with some TI driver ICs that have built in switchmode current limiting, and some fancy microstepping modes.