Buried deep in this article in news.com about challenge-response systems
as a means for fighting spam is this perception of the IETF and spam:
All these should be obvious, but many challenge-response systems just
don't follow them. Fortunately, the Internet Engineering Task Force's
Anti-Spam Research Group is spending some time trying to devise a
reasonable standard.
from: http://news.com.com/2010-1071_3-1009745.html?tag=fd_nc_1
-andy