"Wireless" USB cable, via RF / 3G
Having looked at this, I think it's worth looking at Wireless USB "wire adapters": http://www.usb.org/developers/wusb/docs/presentations/2006/Taipei06_RI_Wire_Adapter_Model.pdf (lots of detail)

However, I don't think those are transparent. If you can't find a transparent one, and it's an absolute requirement, I think you'll have to settle for proxying. Plug in a device on one side, have a "host" read its descriptor and pass it over the wireless, have the proxy present that descriptor on the other side. Store and forward requests; do link-level acknowledgement on the proxy. This should work for HID devices and you can probably make it work for mass storage devices. You will probably have to do special interpretation of some types of message, so build that into your software from the start. This is basically Anindo's solution. I'd estimate it as a good few weeks of software development; once you have the basic XBee devices working you might get better answers on the software stackexchange.

Normal USB hubs are non-buffering and have a very small latency limit of a few 12MHz ticks.