default cipher question

[phr-2002@nightsong.com]

Hi, I noticed the GnuPG 1.0.7 docs say:

      * The default cipher algorithm for encryption is now CAST5,
      default hash algorithm is SHA-1.  This will give us better
      interoperability with other OpenPGP implementations.

My question is, since RFC2440 requires that every OpenPGP
implementation support 3DES, how does using CAST5 as the default
improve interoperability?  If 3DES was the default, wouldn't
GnuPG interoperate with all other conformant implementations?

Can you tell me what other implementations are actually being used
much, and what are their default ciphers?  Is it not realistic to code
an OpenPGP implementation that only supports 3DES?

I'm asking this because I'm planning to submit a block cipher module
to the Python library and was planning to support only AES and DES/3DES.
However, if there are a lot of OpenPGP users using CAST5, maybe that
has to be supported too, since it would be nice to have a Python
implementation of OpenPGP.

>From this point of view, making CAST5 the default instead of 3DES
hurts compatibility rather than helping it.

Thanks / regards

Paul