As a user i really dislike this because more often than not certs are broken because the server is temporarily badly configured (instead of some malicious reason) and i do not even care about it (or even if it was compromised really) as all i care about is read some page's content, connect to some messaging server (Pidgin often had issues with -IIRC- MSN servers), etc.

Of course if it is about, e.g., downloading some application (like an auto-upgrade mechanism) then sure treat it like that. But in other cases let the user decide, even if such sort of decision is made opt-in.