The fix I found was to upgrade CF8's Axis from 1.2.1 to 1.4.
I downloaded Axis 1.4 binaries, stopped CF, copied axis.jar,
wsdl4j-1.5.1.jar,saaj.jar and jaxrpc.jar to Coldfusion's lib folder
and restarted CF.
The ClassCastException didn't arise when testing. One caveat:
Applying Coldfusion 8 updaters will revert Axis back to 1.2.1.
I found this at:
http://tjordahl.blogspot.com/2005/09/upgrading-coldfusion-mx-7-to-axis-13.html