Actually whether you trust apple is irrelevant. The question is whether you have a reasonable expectation of privacy in a conversation you know is being copied to, and read by, a third party. I think the answer to that is probably no as a matter of law. Or, at the very least, the government prosecutors will successfully argue that you don't have a reasonable expectation of privacy in your typings sent to Apple and read by Apple, and thus the government doesn't even need a warrant to listen in (i.e. get copies of that data).
The government can and will demand Apple turn over certain conversations, and Apple will happily oblige (there's a nice tax cut for companies who quietly comply and a nice big audit for companies who complain). I assure you this is already going on. With the advent of Siri, and its central-server processing of all voice transcriptions, the government just got a HUGE new source of domestic intelligence. Oh and yes, IAAL.
So, ironically, the more people are aware of the fact their data is being copied to Apple's main servers, the less of an expectation of privacy we have in said data.