This is a simple and obvious mod to the FTDI board as sold by Sparkfun. I previously ordered the 3.3v version for a project and then discovered that I also needed the 5v version.
As the newer boards contain 5v/3.3v selection pads on the back of the board it was a simple matter to cut the track connecting the common pad to the 3.3v pad and solder a micro slide switch to the pads giving me dual functionality. I used some wire ripped from an IDE cable (the 40 pin ones - 80 wire cables are too fine to work with) - this is a commonly used source of cheap wire as you get 40 from 1 cable! Of course take care with soldering as the pads are quite small.
I simply stuck the switch to the back of the board with a double sided pad and voila - $14 saved!