The NSA's intercepts are a lot different from this. The problem with PRISM and the FISA courts that the NSA is intercepting all the data they think they'll ever need, then they need a warrant to query it. It inverts the intention of what warrants are for, which is to require just cause before any surveillance happens.

With iMessage, if the FBI gives apple a warrant to include their snooping pubkey as an additional encryption endpoint for all messages for a user, by definition it only gives access to messages made from then on, which is in keeping with how I would expect a search warrant to work.