webmail is nice, but with addressbook syncing, it's better. For thunderbird user, waiting for 2.0 releases before starting coding is a good idea, in this version, adressBook is a true sqlite db. For mac user, the adressBook api is easy to access with scripting (like pyobjc), and the iSync api seems to be usable. For that, roundcube should expose some distant API. Is there any choice for that, actually? JSON seems easy and simple. With a json distant API, it will be easy to update the distant adress book.
Mail.app can display picture in mail header, like avatar. It's easy to implement such features.
http://freshmeat.net/projects/mailpictures/
picture and mail adress
I use it with a dirty hack, is it usefull enough to make it cleaner and submit a patch?