Hiroyuki has merged the patches from the sylpheed-gtk2 project and branched them off to a new version 1.9.0 released today, at the same time as the stable Gtk+-1 version 1.0.1. Both new versions carry the same significant difference to 1.0.0, though: They're finally compatible with gpgme > 0.4.
*** Bug 80582 has been marked as a duplicate of this bug. ***
Fixed in CVS. Thank you for reporting.