Considering how obsessed with image, marketing, etc, Apple is, I would be extremely surprised if the Marketing department weren't hugely competent in this matter, including knowing about user-agent strings, logs, etc.

Ask yourself: would Steve Jobs have known about this? Yes, of course. So, why wouldn't the person he hired? This is not a clueless marketing department we're talking about, here... it's probably the most competent marketing departments of any technology company in the world.

Or at this point, they simply don't care, given that:

1. an announcement has been made with a photo hinting at a Retina display

2. it is obvious that Apple is working on iOS 6.0

No one said it was a controlled leak from a marketing department. It could have been managers (at any department, or multiple departments) issuing a statement that the new development software/hardware can be used on the public Internet without worry. I'd imagine the executives or directors would know (or be told) what could happen, and they were comfortable with it.

At that point, it's not a release of information and it's not a leak, but rather a controlled leak.

Occam's razor