At some point in time, Seahorse was making a buggy gpg.conf file which made debuild not use it but ask for the pgp password from the user.
As discussed on #ubuntu-motu, it's OK to fix the buggy config file (if it was generated by Seahorse only).
vadi@ubuntu:~/.gnupg$ egrep ^use-agent $HOME/.gnupg/gpg.conf use-agent vadi@ubuntu:~/.gnupg$ cat gpg.conf # FILE CREATED BY SEAHORSE use-agent vadi@ubuntu:~/.gnupg$