Many legacy programs still use USB license dongles to lock an application to a specific host. This kind of locking makes it difficult to virtualize the server hosting the locked application.
With vSphere 4.1, it is now fully supported to pass through USB to the ESX host, enabling you to plug the license dongle into your ESX host, and attached that to a virtual machine. The USB pass through even works when you vMotion the VM to another ESX host. The vSphere guide has a list of supported USB license key style dongles that have been validated.
However, there is one large caveat with this solution, and that is that if you need to take the ESX host down that is hosting the USB key, for example to do routine maintenance, or during an unplanned HA event, the VM will no longer be able to access the USB key.
This often means customer prefer to stick with the pre-vSphere 4.1 “tried and true” method of using a USB over IP adapter. This method while costing a couple hundred bucks has the advantage the VM still has access to the USB device at all times, assuming the network stays up.