Arduino Wireless USB Cable
I do not think I have ever seen USB over Wifi.. but then question is .. WHY?

Because if you have Wifi connection .. there is no need for USB emulation because WiFi uses TCP/IP- which already sends and receives data very simply.

Arduino Hardware

You already have a choice of Arduino Ethernet which is wired.. sot its not what you looking for. But- There is a network libraru that is easy to use.. and the same library work on the Wireless Shield which use these modules

That all does seem pretty much overkill to buy all of that hardware.



But,

there is alternative. It does not use WiFi but its still wireless and very easy to use from almost any Bluetooth Enabled device. A transceiver like this



Simple just put some power to it. and it has TX/RX pins. Connect to it using a terminal program like putty or terminal on COM port and you can send characters to your arduino and back to your device using normal Arduino Serial commands, without any extra shield or hardware and it runs on most (excluding iPhone-unless you buy a cable) hardware without special programs that have at least BlueeTooth 2.1EDR with SPP profile enabled.