Wow, see this is why it's good to occasionally browse around on the forums.
As extra incentive to those who are musing about attempting this, I'll put on the table $1000 cash or $2000 of ebikes.ca store credit to anyone who successfully makes an open source program that logs/displays CA data onto their iPhone. I'll up that to $1500/$3000 if it also links it with iPhone's GPS data so that there is a location, elevation, slope, and time stamp associated with each CA data point.
Interfacing to external devices to an iPhone isn't trivial, both bluetooth and the proprietary hardware connector would require signing up for the "made for iPhone" program and a whatever expenses they charge. However, as my friend Frank pointed out just today, you can bypass all of that by translating the CA data into a modulated audio signal (FKS) and get the data straight from the microphone jack. As is done here:
http://www.psfk.com/2009/08/prototype-iphone-credit-card-reader.html
I'm serious. Justin