OpenPG/X.509 interoperability

[Christoph Anton Mitterer]

Hi.

This is perhaps a stupid question but how far are these two standards 
interoperable?

I've seen a document that proposes some things in that area 
(http://www.imc.org/ietf-openpgp/mail-archive/msg09930.html).

My question now:
Can X.509 certificates be used to sign/certificate OpenPGP UIDs?
Or can I import the keys as subkeys?
Or are there any other interoperability issues?

What about the two big "free" X.509" suppliers (CACert and Thawte) do 
they sign OpenPGP keys?

Regards,
Christoph.