No one knows for sure. The iMessage app communicates to the Apple servers over SSL, using encryption keys controlled by Apple. This means Apple has the ability, and the according to their EULA, the authority to look at iMessages and the corresponding meta data. To compound matters your AppleID and password are communicated in clear text to Apple servers, making it very simple to match the message to the user. So much for end to end encryption.
For those who don’t know, iMessage is the proprietary text system used by Apple iOS devices. The servers relaying the messages are owned by Apple. The server maintains an open IP connection to all connected iOS devices over an SSL connection. The devices become PUSH clients and the servers use a PUSH notification service.
Since its inception iMessage’s architecture was kept secret by Apple. Researchers from Quarkslab reverse engineered iMessages and published a report on iMessage yesterday. It is important to not the report does not claim Apple is reading iMessages. The report does say Apple has the ability to read iMessages.