Indeed, rumours begat rumours and then that's "fact". Look back at the N9 thread about how the N9 would have miraculous new graphene based technology to render physical keyboards obsolete, definitely come with a dual-core processor and retina display, ...

No, we're asking you to calm down and wait for a definitive answer, and - shock - we're everywhere.

Let's look at MHL rationally, using Wikipedia. Some choice quotes:

Check. Certainly a 3.5mm headphone jack would meet those requirements.

OK, agnostic. But only micro-USB & HDMI connectors used to date. Hmm, doesn't bode well.

That's quite late given what we know of the N9's development.

Nokia: yay! But no actual products or prototypes or statements from Nokia (according to Google) that they're actively working on the technology.

Uh-oh. Exactly as they referred to the N900's TV out.

Let's also think about the circuitry then. Assuming MHL requires 5 pins (4 USB data + ground of a MicroUSB socket), the 3.5mm socket would need to be longer to also maintain compatibility with headsets and headphones. The switching circuitry would be complicated and the socket physically bigger. In a device where size has been optimised as much as possible.