When the phone is connected via USB, it appears as an Ethernet network connection to the device. This means that simply sending data back to the service running on your computer using standard HTTP calls is enough.
This is easy to do by installing a simple web service on your PC and creating a WCF proxy server around it in Silverlight. Then you can just use it like any other web service.
If you are looking for sequential lower-level access to the USB port itself, then, unfortunately, the phone’s APIs do not reveal this functionality.