I'm happy to accept this as the correct explanation, and I'm in no hurry to assign petty motives to Google over this, but in your last link, Matt Cutts says "it can take a few days between when an image appears and when its crawled by the Googlebot." And by "a few days," he appears to literally mean less than a week.

But it's been almost a full month since the WWDC keynote, and nearly two weeks since customers have been posting a barrage of iPhone 4 pictures to the web. So I'm doubtful that Google Images' delayed refresh rate is a sufficient explanation.

Edit: In response to your reply below, you might be right. I've only tried a few searches, but images of major events from the past month seem hard to come by. A search for 'Galaxy S' does give some results from within the past month (the first result was from June 23), as does a search for 'Kindle graphite' (July 2), but presumably refreshes aren't comprehensive. Still, a month-long refresh rate is really surprising. If you need to find images from a recent event, Google Image search may be practically useless.