One thing I have noticed that may help others. FYI, I am running BT4 R1 from a persistent USB install with a ~ two year old Alfa AWUS036H.
If you are running a card with the RTL8187 chipset like the Alfa, Wiffy will switch drivers from the newer "rtl8187" to the older "r8187" for you automatically, as indicated in the above posts. The r8187 driver does not create a 'mon0' interface when going into monitor mode, so you need to change the default monitor interface variables from 'mon0' to 'wlan0' or 'wlan1' within the script itself, or use the command line arguments, ie "./wiffy.sh -i wlan0 -t wlan0" to specify the correct interfaces.
The older r8187 driver also shows all received power levels as 0, which causes an error in the script (usually lines 195 and 196) when it is parsing the power level to decide how to apply the colors -- off the top of my head I believe the error message is "expecting an integer value for variable xxx".