I just got the eZ430 watch the other day and was thinking exactly the same thing!!!
Actually I was thinking of a GPS/Arduino unit somewhere on the bike that
will send real time speed info to be displayed on the watch...
Any idea where to start?
I have even unpacked my watch (no time for a new project yet).
I've given up on the watch (if anybody wants it speak up).
Basically you can only drive it from a USB host like a PC.
There's a lively community but nobody seems to be interested in arduino-type projects.
I think i can be driven by an arduino, but no library exists (that I know of)
so some pretty in depth coding must be done!
But it's possible. And not-so-easy projects are what drives us, right? 
I think you might have the watch backward, if I understand right the devkit lets you change the watch's programming, which can take sensor data and display it.
You would need the Arduino to be attached to one of the sensor transmitters so the watch would receive the value, decode it and display it.
The watch is reprogrammed via USB, but communicates by spy 2 wire or some kind of sensor wireless.
Yes tz, when I say "driven by an arduino" thats what I ment... 
I was responding to:
Basically you can only drive it from a USB host like a PC.
It would be difficult to get a USB host on an arduino, but probably not whatever they are using for the RF sensors.
I think you might have the watch backward, if I understand right the devkit lets you change the watch's programming, which can take sensor data and display it.
http://focus.ti.com/docs/toolsw/folders/print/ez430-chronos.html?DCMP=Chronos&HQ...
You would need the Arduino to be attached to one of the sensor transmitters so the watch would receive the value, decode it and display it.
The watch is reprogrammed via USB, but communicates by spy 2 wire or some kind of sensor wireless.
I'd be delighted to be wrong. What's an example of a sensor transmitter? I looked at the technique being used for the heartrate monitor but it was proprietary hardware and software - not something I could tie in to.
that link didn't seem to work for me by the way.
The CC1110 and or the CC1111 seems to be the chip that does the RF stuff and there are separate devkits for it, one using the USB dongle and it does mention virtual serial port. I don't know about the link - it works from here (in the USA). the CC1110 devkit page is at: http://focus.ti.com/docs/toolsw/folders/print/cc1110-cc1111dk.html You might want to try googling for the watch and or CC1110
Thanks. I did see those chips although not the $650 development kit. I keep an eye on the mailing list to see if anyone gets interested in micro to watch comm but i haven't seen it yet.
Nice color combi with the bike. Very nice gadget you have created. Good luck with the modifications.