I've been looking for some documentation about JCA and came across this tutorial on IBM developerWorks (registration required). From a first pass, it looks pretty good, although I can only seem to be able to print the first three pages of the PDF version.
Out of interest, has anybody ever used JCA to access flat files from within J2EE/EJB? There are a couple of commercial adapters available on Sun's JCA adapters page but I've not been able to find much other than that.