LPT<->USB converters are much harder to use than a built-in LPT port, from what I've heard, because the built-in ports are memory-mapped, so software interfacing is very easy (just read and write to memory locations), whereas the USB ones are not memory-mapped, so you would have to deal with whatever drivers the device uses to communicate with it; ie- a lot more work.

it could be done with a serial port, then you'd just have to use a microcontroller on the recieving end to convert the serial data to parallel outputs. (extremely easy... if mastero or someone else wants to tackle the software stuff, I would be willing to do the microcontroller stuff... mastero, perhaps you could add it as an option in your software?) and interfacing with a USB<->serial converter is the same as interfacing with a built-in serial port AFAIK, since they both just appear as COM ports to software (and COM ports are pretty easy to deal with for programmers, generally). That would make it more adaptable for people who are addicted to USB, especially since a lot of people already have USB hubs up front that it could be plugged into, saving having to run a new cable. And even if you ran a new cable, it would only have to be one wire (use chassis for ground, and power it via a 5v regulator off the closest 12v line)