I went to "You Do It Electronics," the closest thing the Boston area has to a Fry's, and found that they had them. Fortunately, they had them marked incorrectly on their website, $29.90 rather than their normal $49.90, bonus for me as they gave me the website prices when I said that's why I came.
There is no official support for the device under Linux, there is, however, a fairly hackerish USB driver out there. It doesn't fully work, but it was close enough to take a good look. I should be releasing a cleaned up interface class on the LinuxPCRobot.org site sooner or later.
By replacing "usb_interrupt_[read|write]" with "usb_bulk_[read|write]" I was able to get about 1200~1300 read-write cycles out of the unit. This will clearly make the LinuxPCRobot less bandwidth limited on I/O over the parallel port I2C. Also, since USB is handled in hardware, the communication between computer and device takes very little CPU.
The D2A circuit utilizes PWM. An analog signal is generated by a simple R/C circuit fed into an op/amp.
The A2D circuit utilizes a simple op/amp with programmable gain.
It uses a programmed PIC16C745 PIC chip with built in USB, PWM, and A2D converters.
It is clear that Velleman doesn't utilize all that this chip can do, but it is a pretty good start for short money.