I fail to see what benefit there is to making iPhones thinner at this point. And to me it's even more of a negative if the headphone jack has to go. So much so that for once since the original iPhone, I might consider not upgrading if the next model loses the 3.5mm jack.
Even providing an adapter for free wouldn't justify such a decision. Great, now I have to plug this dongle in to use my headphones. How does that help the aesthetic of a thinner phone if I have to carry an extra dongle around now, and basically consider it an additional, more fragile part of my phone?
Keep the phone the same thickness, so the jack doesn't have to go, and work on increasing battery capacity. That would be a worthwhile advancement to me.
Ultimately though, this IS speculation, and not all rumors pan out. I have my doubts that Apple is seriously considering doing away with the jack, even if they are trying to make the phone thinner. the current iPod touch shows that you can still make a thinner device while keeping the jack.