Hi, I wanted to control servos over USB and contacted these guys www.mirrorbow.com
who have a natty little USB IO board. The board itself is spec'd to have 24 digital IO, 2 Analog to digital channels and a PWM, all self powered and controlled over USB (though you need extra power for your add ons like servos). It simulates a com port, which was a boon as its much easier to write a program on a PC to send commands to a com port than to fiddle with DLLs and talk to USB directly.
The good news is that when I asked the tech guy at Mirrorbow about servo control, he already had a prototype which turns one of the ports into an 8 bit servo controller (for 8 servos), so they supplied me with a board with that software in it. Its working great!
If anyones interested in the servo functionality make sure you contact Mirrorbow first as I got the firmware on request.