I have seen many of you using a serial port IBUS interface, but also saw some people having to buy a serial to usb cable to use it. How about one that is already using a usb port? I have one from http://www.eedesignkits.com and it works pretty well! It uses the melexis chip so it has collision detection. Overall I'm happy with the product.
Check it out if you are interested, I think these guys are new!