That’s not quite true. We know that the event will be the public unveiling of iOS8, if you don’t count the developer versions and the public beta. But the size of the phones, and processor they’re based on (most likely the A8) and its exact hardware features, not so much.
If there’s going to be an iWatch announcement, though, bet on it coming at this event. Apple does group its product announcements by platform, so wristware won’t be introduced at any expected iMac refresh, which will be in a separate event later in the year (if at all).
Speaking of the iWatch, 9to5Mac is reporting that Apple has filed trademark applications for Healthkit in the U.S. and Europe:
While the filing in the US (filed July 31) only includes classifications for computer software and covers the Healthkit text, a filing in Europe (published yesterday) extends classifications to include health, fitness, and exercise sensors, medical devices, and watches.