Gábor Hojtsy (one of the Drupal 6 committers and the person behind the localization server module for Drupal) has added today the Greek language in the official drupal.org l10n server, which is currently in beta testing.
Fellow Greek users, if you are interested in contributing with Greek translations, either for Drupal or for contributed modules, you are free to join at http://localize.drupal.org/translate/languages/el. Login with your drupal.org account, join the group and start translating!
Currently, all translations are added as suggestions, which are moderated by the group's moderators (which is currently me, but I am about to add at least one more person). If you already have a translation in gettext format (.po file), you can simply import it.
Let's hope once Drupal 7 hits the streets, Greek users will have the ability to work in their native language.