Having the Certificate Authority private keys would allow an attacker to sign certificates allowing the impersonation of any entity... but it does not make other sessions, boostrapped from other legitimate certificates, transparent to passive eavesdropping.
So no, government pressure on CAs wouldn't make all https like plaintext, but it does make active impersonation possible. Some initiatives like the 'SSL Observatory' or the EFF's 'Sovereign Keys' proposal could make it possible for clients to notice when a fishy new certificate is introduced for a previously-known identity -- much like the 'key has changed' warnings you may have seen when SSHing to a host whose keys have changed since your last session.
So no, government pressure on CAs wouldn't make all https like plaintext, but it does make active impersonation possible. Some initiatives like the 'SSL Observatory' or the EFF's 'Sovereign Keys' proposal could make it possible for clients to notice when a fishy new certificate is introduced for a previously-known identity -- much like the 'key has changed' warnings you may have seen when SSHing to a host whose keys have changed since your last session.