Carl,
When EDE(2) was initially developed, my recollection is that
the intent was to provide a stronger code book than either single DES
or EE. (Having been involved in many of the early DES activties my
recollection is pretty good, but not infalliable.) I would feel
confident in using EDE as a code book for CBC, but if one were to form
a new CBC mode, with each single DES running as a separate encryptor,
I don't feel qualified to evaluate the security of the resulting
system. I would not be comfortable with that latter mode without some
very strong endorsement from folks who are very well versed in the
area. However, I agree that using the EDE as a code book for CBC is
not very attractive for many DES chips. Chips that do onboard CBC
would be penalized by having to switch keys (modern DES chips usually
hold multiple keys so there need not be an external key lodaing
penalty). However, making three passes through the chip with the data
is not very attractive either, given many chip designs.
Steve