Humn, simple requirements....I'd have gone the PIC route, despite having the worst development environment in the world (MPLAB sucks!) that will lead you to pull your hair out, they do have a functional (if very badly written) USB device stack with working CDC implementation, if nothing more you get the ability to use either real RS232 to control your GPIO or a CDC serial port, they even have a reference design which will do exactly what you want.

They have their own C compiler which runs "fully functional" for X months, after which it turns off a few bits and bobs, but the USB reference design still compiles and works perfectly even in the limited mode.

I can't remember what PIC it was that we used on our design, but if you download the microchip USB kit it's the one that they use on the CDC project, the chip is cheap, like a couple of dollars.

Incidentally, we're just finishing off a medical instrument which uses a luminary part, even considering that the chip had a step up in IO from what we'd previously used, we still ended up breaking out the external interface bus and hooking a CPLD up to it, now we have oodles of super fast interrupt driven IO and I also bundled in the logic to control the graphics LCD, the LCD data and instruction registers are just mapped into different memory addresses (twice as the graphics display actually has 2 controllers on it).

Of course, if you didn't baulk at the extra effort of a luminary part (the more advanced ones are QFP's) then you could have your entire development chain in linux, no need to boot into windows for anything...well I say that, stellarisware is provided as a windows installer...maybe I should comment to TI about that.