Just downloaded Picard 1.3 today, and when I ran it I got a code signature error.

"MusicBrainz Picard" can't be opened because it is from an unidentified developer.
Your security preferences allow installation of only apps from the Mac App Store and identified developers.
Safari downloaded this file today at 9:43 PM from ftp.musicbrainz.org.

To fix, you need to make sure you are signed in as a developer with a current certificate when you compile/sign the application.

The workaround for end users is to either change settings to run unsigned applications or to right click on Picard, then choose "Open" to add it to an exception list.