The SPI2UART or MULTIUART open source project is designed to provide additional serial ports for communications with multiple embedded modules such as mobile phone network (GSM), Bluetooth, Wifi etc.
A full build guide is now available via instructables at the link below.
http://www.instructables.com/id/SPI-to-4-x-UART-Bridge-MULTIUART/
A number of boards are currently available on sale via eBay.
An Arduino library and Flowcode component are now available to help driver the board.
