There's a few different options you have. Instrument vendors that have added USB ports to their GPIB instrument have made them compliant with the USBTMC class. You can find out more about this at www.usb.org. It's possible to use VISA to write to USB devices on a lower level. Do a site search for 'VISA USB' for info. You'll also find some additional information on USBTMC. I'm currently using some in-house designed USB instruments that I control with some low level VISA functions. The instruments were designed just for users of LabVIEW or NI's product called TestStand. It would require a user to have VISA installed and while that's not an issue with LabVIEW users, there would be licensing issues if someone were wanting to use your instrument with VB, C++, etc. USBTMC requires VISA as well.
You could also write a dll in a language such as C++ that did the USB RAW communication and provide the api for the dll to all users. This might be the most universal solution. There is a shipping document with LabVIEW called 'Using External Code with LabVIEW' that explains how LabVIEW can call a dll and what the limitations are.
