I thought "Critical" was enough ... see http://issues.cocoondev.org/browse/FOR-443
Well critical may not prevent a release (depends on just how critical, i.e. is it critical for a planned feature or an implemented feature). Blocker on the other hand will prevent a release with that bug.
Blockers also tend to attract allot of attention from the devs with spare time.
Anyway, I'm looking into this now.
Ross
