I use the MOXA brand of USB to Serial hubs. Works great I have 3 DirecTV Sony SAT-B55 receivers under serial control via the MOXA hub.
Their stuff is a little expensive ~$100 range, and you can't buy direct from MOXA, so happy hunting on Froogle for their products.
See their stuff here:
http://www.moxa.com/product/USB_to_Serial_Hubs.htm