Nice job. I have recently started to play with the Arduino. Your library should make it easy to interface with my FT897D which I think uses the same command structure. It is pretty neat to be able to use the standard Arduino I/O to interface directly with the CAT interface.
I am currently going through the Arduino Cookbook to gain some experience with the Arduino board. I have bought a couple of shields (I2C LCD and Ethernet) which are helping me get my feet wet. I combined the I2C and a Temperature sensor and programmed a quick and dirty digital thermometer; which was pretty cool.
I'm looking forward to digging into your library.