The Cypress Semiconductor CY4601 reference design is a 'USB to serial converter' solution. There are a number of such products around - some, like the FTDI solution, are based on an ASIC and don't have a microcontroller at the perpheral end at all. Others, like the Cypress version, use a general-purpose USB peripheral microcontroller with appropriate firmware.
It's surprisingly difficult to emulate a PC serial port with a USB peripheral. People who haven't tried rarely believe that, but it's true for a variety of reasons. Maybe I'll post about that another time. But I think most 'power users' will have come across USB-Serial widgets at one time or another, and the experience is often imperfect.
Anyway, our connection with the product is that a long time ago, we wrote the WDM driver which ships with the Cypress reference design. The version which you can actually download from Cypress is fairly rudimentary - since that particular contract with Cypress came to an end, we have continued to improve the driver and have built a number of custom versions for various other clients. The improvements have tended to be either to application compatibility, or to general robustness. Several versions of the driver have been WHQL signed for clients.
Prior to last week, we thought the current driver was pretty much there...