I noticed that support for GPG/PGP encryption was on the RoundCube roadmap as a planned feature. I, for one, think this would be an awesome feature to have in a webmail client. It does bring up some issues with storing private keys for signing purposes. I guess users would just have to trust their webmail provider with securing their private keys.
One possibility would be to implement the feature in two phases: message encryption first since it relies on public keys only and then message signing which relies on private keys. This project is abandoned http://freshmeat.net/projects/openpgpwebmail/ but might be able to be gutted and reused for the message encryption portion of the problem. It looks like it's under the GNU GPL just as RoundCube is so using code from it shouldn't be a problem.
If I get some time I may try and see if I can graft some GPG/PGP features in to RoundCube. My initial idea would be to make these features dependent on GnuPG. Thoughts?
-- Bradley Holt