Just this afternoon I was looking up the wifi module Async use.
Its the Microchip zg2100mmc
Its available in the UK for £18.38 +vat.
The external antenna version is available as well for the same price
I think RS components have them listed as well.
It needs 3.3v and a breakout board unless you created your own board.
Does the schematic show level shifers as well?
But Im sure you could use the Wishield libraries once you have it hooked up.
Gordon